Explicitly state supported Linux versions

Add META tags Linux-Maximum and Linux-Minimum.

One pain point for package maintainers is ensuring the compatibility of
the packaged version of ZFS with the Linux kernel. By providing an
authoritative compatibility guide in the source tree, maintainers can
automate compatibility checks.

Additionally, increase META string extraction specificity.
configure.ac finds Name and Version by a very simple `grep`, which might
conceivably find other fields. Require the string be at the beginning of
a line, and be followed by a colon to avoid such confusions.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Giuseppe Di Natale <guss80@gmail.com>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Antonio Russo <antonio.e.russo@gmail.com>
Closes #7571

META

@@ -6,3 +6,5 @@ Release:      1
 Release-Tags:  relext
 License:       CDDL
 Author:        OpenZFS on Linux
+Linux-Maximum: 4.16
+Linux-Minimum: 2.6.32

README.md

@@ -26,3 +26,6 @@ We have a separate document with [contribution guidelines](./.github/CONTRIBUTIN
 
 ZFS on Linux is released under a CDDL license.  
 For more details see the NOTICE, LICENSE and COPYRIGHT files; `UCRL-CODE-235197`
+
+# Supported Kernels
+  * The `META` file contains the officially recognized supported kernel versions.

configure.ac

@@ -30,8 +30,8 @@
  * CDDL HEADER END
  */
 
-AC_INIT(m4_esyscmd(grep Name META | cut -d ':' -f 2 | tr -d ' \n'),
-	m4_esyscmd(grep Version META | cut -d ':' -f 2 | tr -d ' \n'))
+AC_INIT(m4_esyscmd(grep ^Name: META | cut -d ':' -f 2 | tr -d ' \n'),
+	m4_esyscmd(grep ^Version: META | cut -d ':' -f 2 | tr -d ' \n'))
 AC_LANG(C)
 ZFS_AC_META
 AC_CONFIG_AUX_DIR([config])