osd: do not request backlog from peers with empty pg
osd: do not request backlog from peers with empty pg

This avoids stalling out peering, because the peer just responds with
another 'empty' PG::Info in response (which we already have).

Signed-off-by: Sage Weil <6dd34506bbd5e58221bbb3e4732d97c91f02277b@newdream.net>

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