ceph: fix a NULL vs IS_ERR() check when calling ceph_lookup_inode()

The ceph_lookup_inode() function returns error pointers.  It never
returns NULL.

Fixes: aa87052d ("ceph: fix incorrectly showing the .snap size for stat")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: Xiubo Li <xiubli@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>