bcachefs: Convert more locking code to btree_bkey_cached_common

bcachefs: Convert more locking code to btree_bkey_cached_common

Ideally, all the code in btree_locking.c should be converted, but then
we'd want to convert btree_path to point to btree_key_cached_common too,
and then we'd be in for a much bigger cleanup - but a bit of incremental
cleanup will still be helpful for the next patches.

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