mds: fix crash in hash_into_rank_bucket() when max_mds is 0

When a CephFS cluster is paused (e.g. via `ceph fs set <fs> down true`
or `ceph fs pause`) the MDS map's max_mds is set to 0.  Any subsequent
call to hash_into_rank_bucket() with max_mds == 0 triggers a crash:
the jump-consistent-hash loop never executes (j starts at 0, condition
j < max_mds is immediately false), leaving b = -1, so the final
assert(result >= 0 && result < max_mds) aborts the daemon.

Fixes: https://tracker.ceph.com/issues/76059
Signed-off-by: chungfengz <chungfengz@synology.com>