ceph: stop retrying the request when exceeding 256 times

The type of 'r_attempts' in kernel 'ceph_mds_request' is 'int',
while in 'ceph_mds_request_head' the type of 'num_retry' is '__u8'.
So in case the request retries exceeding 256 times, the MDS will
receive a incorrect retry seq.

In this case it's ususally a bug in MDS and continue retrying the
request makes no sense. For now let's limit it to 256. In future
this could be fixed in ceph code, so avoid using the hardcode here.

Signed-off-by: Xiubo Li <xiubli@redhat.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>