More bcachefs bugfixes for 6.7:

Just a few fixes: besides a few one liners, we have a fix for snapshots
+ compression where the extent update path didn't account for the fact
that with snapshots, we might split an existing extent into three, not
just two; and a small fixup for promotes which were broken by the recent
changes in the data update path to correctly take into account device
durability.
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