osd: no osd_lock for finish_splits

osd: no osd_lock for finish_splits

This used to protect the pg registration probably?  There is no need for
it now.

More importantly, having it here can cause a deadlock when we are holding
osd_lock and blocking on wait_min_pg_epoch(), because a PG may need to
finish splitting to advance and then merge with a peer.  (The wait won't
block on *this* PG since it isn't registered in the shard yet, but it
will block on the merge peer.)

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