bcachefs: bch_dev_usage_full

bcachefs: bch_dev_usage_full

All the fastpaths that need device usage don't need the sector totals or
fragmentation, just bucket counts.

Split bch_dev_usage up into two different versions, the normal one with
just bucket counts.

This is also a stack usage improvement, since we have a bch_dev_usage on
the stack in the allocation path.

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