bcachefs: Sort & deduplicate updates in bch2_trans_update()

bcachefs: Sort & deduplicate updates in bch2_trans_update()

Previously, when doing multiple update in the same transaction commit
that overwrote each other, we relied on doing the updates in the same
order as the bch2_trans_update() calls in order to get the correct
result. But that wasn't correct for triggers; bch2_trans_mark_update()
when marking overwrites would do the wrong thing because it hadn't seen
the update that was being overwritten.

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