MOSDMap: reencode maps if target doesn't have OSDMAP_ENC

Reencode both full and incremental maps if target doesn't know how to
decode OSDMAP_ENC maps (CEPH_FEATURE_OSDMAP_ENC bit is not set).  This
fixes a compatibility bug that was introduced in 3d7c69fb0986 ("OSDMap:
add a CEPH_FEATURE_OSDMAP_ENC feature, and use new encoding").

Signed-off-by: Ilya Dryomov <ilya.dryomov@inktank.com>
Merge pull request #1126 from ceph/wip-osdmapenc-fix

MOSDMap: reencode maps if target doesn't have OSDMAP_ENC

Reviewed-by: Sage Weil <sage@inktank.com>