bcachefs: Make various async objs visible in debugfs

Add async objs list for
- promote_op
- bch_read_bio
- btree_read_bio
- btree_write_bio

This gets us introspection on in-flight async ops, and because under the
hood it uses fast_lists (percpu slot buffer on top of a radix tree),
it'll be fast enough to enable in production.

This will be very helpful for debugging "something got stuck" issues,
which have been cropping up from time to time (in the CI, especially
with folio writeback).

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

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