bcachefs: Fragmentation LRU

bcachefs: Fragmentation LRU

Now that we have much more efficient updates to the LRU btree, this
patch adds a new LRU that indexes buckets by fragmentation.

This means copygc no longer has to scan every bucket to find buckets
that need to be evacuated.

Changes:
 - A new field in bch_alloc_v4, fragmentation_lru - this corresponds to
   the bucket's position in the fragmentation LRU. We add a new field
   for this instead of calculating it as needed because we may make the
   fragmentation LRU optional; this field indicates whether a bucket is
   on the fragmentation LRU.

   Also, zoned devices will introduce variable bucket sizes; explicitly
   recording the LRU position will be safer for them.

 - A new copygc path for using the fragmentation LRU instead of
   scanning every bucket and building up an in-memory heap.

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