Configure --with-spl{-obj} auto-detect cleanup

Because the install location for the spl/zfs-devel headers was
changed we need to refresh the auto-detect code.  Note that
for packaging which already explicitly calls --with-spl{-obj}
nothing has changed.

The updated code is now structured like that in ZFS_AC_KERNEL
and should be cleaner and easier to maintain.  In addition,
it's stricter about detecting a valid source and object
directory.  It requires:

* The source directory contains the file 'spl.release'
* The object directory contains the file 'spl_config.h'
* The following paths will be checked.  Notice the /var/lib/
  and /usr/src paths require that the spl and zfs version be
  matched.  This is done to prevent accidentally mixing releases.

        dnl # 1) /var/lib/dkms/spl/<version>/build
        dnl # 2) /usr/src/spl-<version>/<kernel-version>
        dnl # 3) /usr/src/spl-<version>
        dnl # 4) ../spl
        dnl # 5) /usr/src/kernels/<kernel-version>

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>

config/kernel.m4

@@ -292,45 +292,30 @@ AC_DEFUN([ZFS_AC_SPL], [
 		[Path to spl build objects]),
 		[splbuild="$withval"])
 
+	dnl #
+	dnl # The existence of spl.release.in is used to identify a valid
+	dnl # source directory.  In order of preference:
+	dnl #
+	splsrc0="/var/lib/dkms/spl/${VERSION}/build"
+	splsrc1="/usr/src/spl-${VERSION}/${LINUX_VERSION}"
+	splsrc2="/usr/src/spl-${VERSION}"
+	splsrc3="../spl/"
+	splsrc4="$LINUX"
 
 	AC_MSG_CHECKING([spl source directory])
-	AS_IF([test -z "$splsrc"], [
+	AS_IF([test -z "${splsrc}"], [
-		dnl #
+		AS_IF([ test -e "${splsrc0}/spl.release.in"], [
-		dnl # Look in the standard development package location
+			splsrc=${splsrc0}
-		dnl #
+		], [ test -e "${splsrc1}/spl.release.in"], [
-		sourcelink=`ls -1d /usr/src/spl-*/${LINUX_VERSION} \
+			splsrc=${splsrc1}
-		            2>/dev/null | tail -1`
+		], [ test -e "${splsrc2}/spl.release.in"], [
-
+			splsrc=${splsrc2}
-		dnl #
+		], [ test -e "${splsrc3}/spl.release.in"], [
-		dnl # Look in the DKMS source location
+			splsrc=$(readlink -f "${splsrc3}")
-		dnl #
+		], [ test -e "${splsrc4}/spl.release.in" ], [
-		AS_IF([test -z "$sourcelink" || test ! -e $sourcelink/spl_config.h], [
+			splsrc=${splsrc4}
-			sourcelink=`ls -1d /var/lib/dkms/spl/*/build \
-			            2>/dev/null | tail -1`
-		])
-
-		dnl #
-		dnl # Look in the parent directory
-		dnl #
-		AS_IF([test -z "$sourcelink" || test ! -e $sourcelink/spl_config.h], [
-			sourcelink=../spl
-		])
-
-		dnl #
-		dnl # Look in the kernel directory
-		dnl #
-		AS_IF([test -z "$sourcelink" || test ! -e $sourcelink/spl_config.h], [
-			sourcelink="$LINUX"
-		])
-
-		AS_IF([test -e $sourcelink/spl_config.h], [
-			splsrc=`readlink -f ${sourcelink}`
 		], [
-			AC_MSG_RESULT([Not found])
+			splsrc="[Not found]"
-			AC_MSG_ERROR([
-	*** Please make sure the spl devel package for your distribution
-	*** is installed then try again.  If that fails you can specify the
-	*** location of the spl source with the '--with-spl=PATH' option.])
 		])
 	], [
 		AS_IF([test "$splsrc" = "NONE"], [
@@ -340,11 +325,38 @@ AC_DEFUN([ZFS_AC_SPL], [
 	])
 
 	AC_MSG_RESULT([$splsrc])
+	AS_IF([ test ! -e "$splsrc/spl.release.in"], [
+		AC_MSG_ERROR([
+	*** Please make sure the kmod spl devel package for your distribution
+	*** is installed then try again.  If that fails you can specify the
+	*** location of the spl source with the '--with-spl=PATH' option.])
+	])
+
+	dnl #
+	dnl # The existence of the spl_config.h is used to identify a valid
+	dnl # spl object directory.  In many cases the object and source
+	dnl # directory are the same, however the objects may also reside
+	dnl # is a subdirectory named after the kernel version.
+	dnl #
 	AC_MSG_CHECKING([spl build directory])
 	AS_IF([test -z "$splbuild"], [
-		splbuild=${splsrc}
+		AS_IF([ test -e "${splsrc}/${LINUX_VERSION}/spl_config.h" ], [
+			splbuild="${splsrc}/${LINUX_VERSION}"
+		], [ test -e "${splsrc}/spl_config.h" ], [
+			splbuild="${splsrc}"
+		], [
+			splbuild="[Not found]"
+		])
 	])
+
 	AC_MSG_RESULT([$splbuild])
+	AS_IF([ ! test -e "$splbuild/spl_config.h"], [
+		AC_MSG_ERROR([
+	*** Please make sure the kmod spl devel <kernel> package for your
+	*** distribution is installed then try again.  If that fails you
+	*** can specify the location of the spl objects with the
+	*** '--with-spl-obj=PATH' option.])
+	])
 
 	AC_MSG_CHECKING([spl source version])
 	AS_IF([test -r $splbuild/spl_config.h &&