osd/ReplicatedPG: initialize new_backfill in ctor

*** CID 1260213:  Uninitialized scalar field  (UNINIT_CTOR)
/osd/ReplicatedPG.cc: 1242 in ReplicatedPG::ReplicatedPG(OSDService *,
std::tr1::shared_ptr<const OSDMap>, const PGPool &, spg_t)()
1236       snap_trimmer_machine(this)
1237     {
1238       missing_loc.set_backend_predicates(
1239         pgbackend->get_is_readable_predicate(),
1240         pgbackend->get_is_recoverable_predicate());
1241       snap_trimmer_machine.initiate();
>>>     CID 1260213:  Uninitialized scalar field  (UNINIT_CTOR)
>>>     Non-static class member "new_backfill" is not initialized in this
constructor nor in any functions that it calls.
1242     }
1243
1244     void ReplicatedPG::get_src_oloc(const object_t& oid, const
object_locator_t& oloc, object_locator_t& src_oloc)
1245     {
1246       src_oloc = oloc;
1247       if (oloc.key.empty())

Signed-off-by: Sage Weil <sage@redhat.com>