bcachefs: Update transactional triggers interface to pass old & new keys

bcachefs: Update transactional triggers interface to pass old & new keys

This is needed to fix a bug where we're overflowing iterators within a
btree transaction, because we're updating the stripes btree (to update
block counts) and the stripes btree trigger is unnecessarily updating
the alloc btree - it doesn't need to update the alloc btree when the
pointers within a stripe aren't changing.

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