The trigger for stripe keys is shortly going to need both the old and
the new key passed to the trigger - this patch does that rework.

For now, this just changes the in memory triggers, and this doesn't
change how extent triggers work.
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>