osd: only verfy OSDMap crc if it is known

Only verify we encode a full map with the correct CRC if we actually
have the value in the Incremental.  Otherwise, any map from an old
mon will fail.  And we'll try to request a full map with a message the
old mon doesn't understand.

Signed-off-by: Sage Weil <sage@redhat.com>
Merge pull request #3210 from ceph/wip-osdmap

osd: only verify OSDMap crc if it is known

Reviewed-by: Loic Dachary <ldachary@redhat.com>