osd/PGLog: reset complete_to when appending lost_delete entries

osd/PGLog: reset complete_to when appending lost_delete entries

Since lost_deletes queue recovery directly, and don't go through
activate_not_complete(), our complete_to iterator may still point at
log.end() (a list iterator pointing to .end() will still point to
.end() after a push_back().). Reset it to point before these new
lost_delete entries. This is needed now that lost_deletes are
performed during recovery, instead of inline when merging logs.

Signed-off-by: Josh Durgin <jdurgin@redhat.com>