openssd: fix locking for assumptions that no longer holds

Forgot the assumptions I made around trans_map and block->invalid_pages, so
that I made several updates in bulk, I create a race-condition, which
unfortunately lead to pages being missing in-flight.
openssd: don't bug on no invalid pages.

We might come to a point where gc is trying to gc pages containing no
invalid pages. This shouldn't be considered a bug, but simply return
instead.