Linux 7.0: add shims for the fs_context-based mount API

The traditional mount API has been removed, so detect when its not
available and instead use a small adapter to allow our existing mount
functions to keep working.

Sponsored-by: TrueNAS
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rob Norris <rob.norris@truenas.com>
Closes #18216

config/kernel-fst-mount.m4

@@ -4,6 +4,10 @@ dnl # 2.6.38 API change
 dnl # The .get_sb callback has been replaced by a .mount callback
 dnl # in the file_system_type structure.
 dnl #
+dnl # 7.0 API change
+dnl # The .mount callback has been removed, requiring all mount work
+dnl # to be done through the "new" mount API introduced in 5.2.
+dnl #
 AC_DEFUN([ZFS_AC_KERNEL_SRC_FST_MOUNT], [
         ZFS_LINUX_TEST_SRC([file_system_type_mount], [
                 #include <linux/fs.h>
@@ -25,7 +29,8 @@ AC_DEFUN([ZFS_AC_KERNEL_FST_MOUNT], [
         AC_MSG_CHECKING([whether fst->mount() exists])
         ZFS_LINUX_TEST_RESULT([file_system_type_mount], [
                 AC_MSG_RESULT(yes)
+		AC_DEFINE(HAVE_FST_MOUNT, 1, [fst->mount() exists])
         ],[
-		ZFS_LINUX_TEST_ERROR([fst->mount()])
+		AC_MSG_RESULT(no)
         ])
 ])