gfs2_meta: ->mount() can get NULL dev_name

get in sync with mount_bdev() handling of the same

Reported-by: syzbot+c54f8e94e6bba03b04e9@syzkaller.appspotmail.com
Cc: stable@vger.kernel.org
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/gfs2/ops_fstype.c

@@ -1333,6 +1333,9 @@ static struct dentry *gfs2_mount_meta(struct file_system_type *fs_type,
 	struct path path;
 	int error;
 
+	if (!dev_name || !*dev_name)
+		return ERR_PTR(-EINVAL);
+
 	error = kern_path(dev_name, LOOKUP_FOLLOW, &path);
 	if (error) {
 		pr_warn("path_lookup on %s returned error %d\n",