osd: fix peer no missing optimization

osd: fix peer no missing optimization

This shortcut was broken: we need to populate peer_missing with missing
objects in terms of the master log, not the peer's log (which may be old
or even divergent).  This shortcut _only_ makes sense when the peer has
no missing in terms of a log that is perfectly up to date (i.e. matches
our last_update).

Reported-by: Henry Chang <henry.cy.chang@gmail.com>
Signed-off-by: Sage Weil <sage.weil@dreamhost.com>