ceph: fix sizeof(struct tYpO *) typo

struct ceph_xattr -> struct ceph_inode_xattr

Reported-by: Toralf Förster <toralf.foerster@gmx.de>
Signed-off-by: Ilya Dryomov <ilya.dryomov@inktank.com>
Reviewed-by: Alex Elder <elder@linaro.org>