client: check if a mds rank is `up` before fetching connection addr

client: check if a mds rank is `up` before fetching connection addr

Client segfaults when trying to infer which mds rank a connection
reset call is coming from. It does this by iterating `mds_sessions`
and checking the mds addr (in mdsmap) to the `Connection *`.

However, cases where the mds is blocklisted, the client receives
an updated mdsmap in which the corresponding mds is not in `up`
set thereby resulting in a segfault when calling `mdsmap->get_addrs`
since it expects that the mds should be in `up` state.

Note that this leaves the `Connection *` as it is and does not clean
it up. That needs to fixed separately probably by maintaining a map
of `Connection *` to mds rank for lookup.

Fixes: http://tracker.ceph.com/issues/50530
Signed-off-by: Venky Shankar <vshankar@redhat.com>