bcachefs: Advance bch_alloc.oldest_gen if no stale pointers

bcachefs: Advance bch_alloc.oldest_gen if no stale pointers

Now that we've got cached backpointers and aren't leaving around stale
pointers on bucket invalidation, we no longer need the periodic (rare)
gc_gens - which recalculates each bucket's oldest gen to avoid wraparound.

We can't delete that code because we've got to support existing
filesystems that will still have stale pointers, but this gets rid of
another scalability limit.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>