bcachefs: support btree updates of prejournaled keys

bcachefs: support btree updates of prejournaled keys

Introduce support for prejournaled key updates. This allows a
transaction to commit an update for a key that already exists (and
is pinned) in the journal. This is required for btree write buffer
updates as the current scheme of journaling both on write buffer
insertion and write buffer (slow path) flush is unsafe in certain
crash recovery scenarios.

Create a small trans update wrapper to pass along the seq where the
key resides into the btree_insert_entry. From there, trans commit
passes the seq into the btree insert path where it is used to manage
the journal pin for the associated btree leaf.

Note that this patch only introduces the underlying mechanism and
otherwise includes no functional changes.

Signed-off-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>