mon/OSDMonitor: fix osdmap encode feature logic

If we are encoding a full map based on an old Incremental that does not
encode the features, fall back to the quorum features or (barring that)
all features.  Do *not* do no features or else we will end up with
encode_client_old which does not even include the extended info and will
cause the mon to crash when decoding.

This was observed when upgading a 0.76 cluster to 0.77 (all mons stopped,
upgraded, and then started)

Reported-by: Aaron Ten Clay <aarontc@aarontc.com>
Signed-off-by: Sage Weil <sage@inktank.com>
Merge pull request #1308 from ceph/wip-osdmap-inc

mon/OSDMonitor: fix osdmap encode feature logic

Reviewed-by: Greg Farnum <greg@inktank.com>