crimson/osd: complete PGAdvanceMap's pg-deleted path properly

crimson/osd: complete PGAdvanceMap's pg-deleted path properly

When the pg has been deleted while PGAdvanceMap was queued, start()
takes an early return and skips the map-advance loop. The PeeringCtx
handed to the operation may already carry a transaction and queued
peering messages, so returning without calling complete_rctx() would
drop them. Dispatch the rctx before bailing out.

Also leave the PGPeeringPipeline::process stage via handle.complete()
instead of relying on the exit_handle defer's handle.exit(). The
pg-deleted path is a graceful completion, not an op failure, so it
should match the normal completion path; handle.exit() is documented
for the failure case only.

Signed-off-by: Kefu Chai <tchaikov@gmail.com>