Use requested kernel for dkms builds

The --with-linux and --with-linux-obj options must be specified
as part of the dkms build otherwise the package will be built
against the running kernel.

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

scripts/dkms.mkconf

@@ -25,6 +25,8 @@ PACKAGE_CONFIG="${pkgcfg}"
 PRE_BUILD="configure
   --prefix=/usr
   --with-config=kernel
+  --with-linux=\${kernel_source_dir}
+  --with-linux-obj=\${kernel_source_dir}
   --with-spl=\${dkms_tree}/spl/\${PACKAGE_VERSION}/build
   --with-spl-obj=\${dkms_tree}/spl/\${PACKAGE_VERSION}/\${kernelver}/\${arch}
   --with-spl-timeout=300