bcachefs: Perf improvements for bch_alloc_read()

bcachefs: Perf improvements for bch_alloc_read()

On large filesystems reading in the alloc info takes a significant
amount of time. But we don't need to be calling into the fully general
bch2_mark_key() path, just open code what we need in
bch2_alloc_read_fn().

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