ceph: stop forwarding the request when exceeding 256 times

The type of 'num_fwd' in ceph 'MClientRequestForward' is 'int32_t',
while in 'ceph_mds_request_head' the type is '__u8'. So in case
the request bounces between MDSes exceeding 256 times, the client
will get stuck.

In this case it's ususally a bug in MDS and continue bouncing the
request makes no sense.

URL: https://tracker.ceph.com/issues/55130


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