Support idmapped mount

Adds support for idmapped mounts.  Supported as of Linux 5.12 this 
functionality allows user and group IDs to be remapped without changing 
their state on disk.  This can be useful for portable home directories
and a variety of container related use cases.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Signed-off-by: Youzhong Yang <yyang@mathworks.com>
Closes #12923
Closes #13671

module/os/linux/zfs/zpl_super.c

@@ -374,7 +374,11 @@ const struct super_operations zpl_super_operations = {
 struct file_system_type zpl_fs_type = {
 	.owner			= THIS_MODULE,
 	.name			= ZFS_DRIVER,
+#if defined(HAVE_IDMAP_MNT_API)
+	.fs_flags		= FS_USERNS_MOUNT | FS_ALLOW_IDMAP,
+#else
 	.fs_flags		= FS_USERNS_MOUNT,
+#endif
 	.mount			= zpl_mount,
 	.kill_sb		= zpl_kill_sb,
 };