btrfs: periodic block_group reclaim

We currently employ a edge-triggered block group reclaim strategy which
marks block groups for reclaim as they free down past a threshold.

With a dynamic threshold, this is worse than doing it in a
level-triggered fashion periodically. That is because the reclaim
itself happens periodically, so the threshold at that point in time is
what really matters, not the threshold at freeing time. If we mark the
reclaim in a big pass, then sort by usage and do reclaim, we also
benefit from a negative feedback loop preventing unnecessary reclaims as
we crunch through the "best" candidates.

Since this is quite a different model, it requires some additional
support. The edge triggered reclaim has a good heuristic for not
reclaiming fresh block groups, so we need to replace that with a typical
GC sweep mark which skips block groups that have seen an allocation
since the last sweep.

Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Boris Burkov <boris@bur.io>
Signed-off-by: David Sterba <dsterba@suse.com>