Add missing zfs_exit() when snapdir is disabled (openzfs#16912)

zfs_vget doesn't zfs_exit when erroring out due to snapdir
being disabled.

Signed-off-by: Andrew Walker <awalker@ixsystems.com>
Reviewed-by: @bmeagherix
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Ameer Hamza <ahamza@ixsystems.com>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>

module/os/linux/zfs/zfs_vfsops.c

@@ -1702,13 +1702,14 @@ zfs_vget(struct super_block *sb, struct inode **ipp, fid_t *fidp)
 	/* A zero fid_gen means we are in the .zfs control directories */
 	if (fid_gen == 0 &&
 	    (object == ZFSCTL_INO_ROOT || object == ZFSCTL_INO_SNAPDIR)) {
-		*ipp = zfsvfs->z_ctldir;
-		ASSERT(*ipp != NULL);
-
 		if (zfsvfs->z_show_ctldir == ZFS_SNAPDIR_DISABLED) {
+			zfs_exit(zfsvfs, FTAG);
 			return (SET_ERROR(ENOENT));
 		}
 
+		*ipp = zfsvfs->z_ctldir;
+		ASSERT(*ipp != NULL);
+
 		if (object == ZFSCTL_INO_SNAPDIR) {
 			VERIFY(zfsctl_root_lookup(*ipp, "snapshot", ipp,
 			    0, kcred, NULL, NULL) == 0);