osd: fix the mem leak of RepGather

osd: fix the mem leak of RepGather

ReplicatedPG::new_repop() returns a pointer to RepGather with two refcounts,
one is held by ReplicatedPG::repop_queue, the other is supposed to be
held by the caller of this function. but it's caller
ReplicatedPG::submit_log_entries() assigns it to a
boost::intrusive_ptr<RepGather>() directly, why by default add_ref() in
its constructor. this makes the refcount 3. that's why we have a leak of
RepGather in `ReplicatedPG::new_repop(ObcLockManager&&,
boost::optional<std::function<void ()>>&&)`.

Fixes: http://tracker.ceph.com/issues/16801
Signed-off-by: Kefu Chai <kchai@redhat.com>