bcachefs: bch2_btree_path_peek_slot_exact()

bcachefs: bch2_btree_path_peek_slot_exact()

When we start using the key cache for inodes again, it'll be possible
for bch2_btree_path_peek_slot() to return a key in a different snapshot
with a key cache path.

This isn't what we want when triggers are checking what they're
overwriting, so introduce a new helper for the commit path.

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