osd: heartbeat with packets large enough to require working jumbo frames.

osd: heartbeat with packets large enough to require working jumbo frames.

We get periodic reports that users somehow misconfigure one of their switches
so that it drops jumbo frames, yet the servers are still passing them along. In
that case, MOSDOp messages generally don't get through because they are much
larger than the 1500-byte non-jumbo limit, but the MOSDPing messages have kept
going (as they are very small and dispatched independently, even when the
server is willing to make jumbo frames). This means peer OSDs won't mark down
the ones behind the broken switch, despite all IO hanging.
Push the MOSDPing message size over the 1500-byte limit so that anybody in
this scenario will see the OSDs stuck behind a bad switch get marked down.

Fixes: http://tracker.ceph.com/issues/20087
Signed-off-by: Greg Farnum <gfarnum@redhat.com>