osd/PGLog: should not rollback further than deleted object version

osd/PGLog: should not rollback further than deleted object version

When a deleted object becomes a divergent entry in the pg log,
we should not be able to rollback to a version of the deleted
object that doesn't exist.

To avoid this, we need to preserve the original crt of the pg log,
before we update it in rewind_from_head() and use that to decide whether
we can rollback or not in _merge_object_divergent_entries().

Fixes: http://tracker.ceph.com/issues/36739
Signed-off-by: Neha Ojha <nojha@redhat.com>