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>