messages: provide an explicit COMPAT_VERSION in MMDSBeacon

We weren't setting an explicit compat version and so the Message
was setting it to the head version, breaking communication with
older daemons.

Fixes: #9657
Signed-off-by: Greg Farnum <greg@inktank.com>
Merge pull request #2640 from ceph/wip-9657

messages: provide an explicit COMPAT_VERSION in MMDSBeacon

Reviewed-by: Sage Weil <sage@redhat.com>