osd: dispatch_context and queue split finish on early bail-out

osd: dispatch_context and queue split finish on early bail-out

If we bail out of advance_pg early because there is an upcoming merge, we
still need to dispatch_context() on rctx before we drop the PG lock.  And
the rctx that we submit needs to include the on_applied finisher comit
to call _finish_splits.

This is noticeable (at least) when there is a split and merge that are
both known.  When we process the split, the new child is added to new_pgs.
When we get to the merge epoch, we stop early and take the bail-out
path.

Fix by adding a dispatch_context call for this path.  And further make sure
that both dispatch_context callers in this function queue up the
new_pgs event.

Fixes: https://tracker.ceph.com/issues/43825
Signed-off-by: Sage Weil <sage@redhat.com>