Linux 6.3 compat: idmapped mount API changes

Linux kernel 6.3 changed a bunch of APIs to use the dedicated idmap 
type for mounts (struct mnt_idmap), we need to detect these changes 
and make zfs work with the new APIs.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Youzhong Yang <yyang@mathworks.com>
Closes #14682

module/os/linux/zfs/zfs_ioctl_os.c

@@ -282,6 +282,8 @@ zfsdev_detach(void)
 #define	ZFS_DEBUG_STR	""
 #endif
 
+zidmap_t *zfs_init_idmap;
+
 static int
 openzfs_init_os(void)
 {
@@ -305,6 +307,8 @@ openzfs_init_os(void)
 	printk(KERN_NOTICE "ZFS: Posix ACLs disabled by kernel\n");
 #endif /* CONFIG_FS_POSIX_ACL */
 
+	zfs_init_idmap = (zidmap_t *)zfs_get_init_idmap();
+
 	return (0);
 }