mon: test if gid exists in pending for prepare_beacon

mon: test if gid exists in pending for prepare_beacon

If it does not, send a null map. Bug introduced by
624efc64323f99b2e843f376879c1080276e036f which made preprocess_beacon only look
at the current fsmap (correctly). prepare_beacon relied on preprocess_beacon
doing that check on pending.

Running:

    while sleep 0.5; do bin/ceph mds fail 0; done

is sufficient to reproduce this bug. You will see:

    2018-09-07 15:33:30.350 7fffe36a8700  5 mon.a@0(leader).mds e69 preprocess_beacon mdsbeacon(24412/a up:reconnect seq 2 v69) v7 from mds.0 127.0.0.1:6813/2891525302 compat={},rocompat={},incompat={1=base v0.20,2=client writeable ranges,3=default file layouts on dirs,4=dir inode in separate object,5=mds uses versioned encoding,6=dirfrag is stored in omap,8=no anchor table,9=file layout v2,10=snaprealm v2}
    2018-09-07 15:33:30.350 7fffe36a8700 10 mon.a@0(leader).mds e69 preprocess_beacon: GID exists in map: 24412
    2018-09-07 15:33:30.350 7fffe36a8700  5 mon.a@0(leader).mds e69 _note_beacon mdsbeacon(24412/a up:reconnect seq 2 v69) v7 noting time
    2018-09-07 15:33:30.350 7fffe36a8700  7 mon.a@0(leader).mds e69 prepare_update mdsbeacon(24412/a up:reconnect seq 2 v69) v7
    2018-09-07 15:33:30.350 7fffe36a8700 12 mon.a@0(leader).mds e69 prepare_beacon mdsbeacon(24412/a up:reconnect seq 2 v69) v7 from mds.0 127.0.0.1:6813/2891525302
    2018-09-07 15:33:30.350 7fffe36a8700 15 mon.a@0(leader).mds e69 prepare_beacon got health from gid 24412 with 0 metrics.
    2018-09-07 15:33:30.350 7fffe36a8700  5 mon.a@0(leader).mds e69 mds_beacon mdsbeacon(24412/a up:reconnect seq 2 v69) v7 is not in fsmap (state up:reconnect)

in the mon leader log. The last line indicates the problem was safely handled.

Fixes: http://tracker.ceph.com/issues/35848
Signed-off-by: Patrick Donnelly <pdonnell@redhat.com>