osd: fix compensation for bad last_complete
osd: fix compensation for bad last_complete

If the peer has a last_complete below their tail, we can get by with our
log (without backlog) if our tail if _before_ their last_complete, not
after.  Otherwise, we need a backlog!

Signed-off-by: Sage Weil <c5b6ff1fd4f0bde10db63d7a3e07166cf6c4229f@dreamhost.com>

ajnelson/ceph,ajnelson/ceph,ajnelson/ceph,ajnelson/ceph,ajnelson/ceph,ajnelson/ceph