bcachefs: use prejournaled key updates for write buffer flushes

bcachefs: use prejournaled key updates for write buffer flushes

The write buffer mechanism journals keys twice in certain
situations. A key is always journaled on write buffer insertion, and
is potentially journaled again if a write buffer flush falls into
either of the slow btree insert paths. This has shown to cause
journal recovery ordering problems in the event of an untimely
crash.

For example, consider if a key is inserted into index 0 of a write
buffer, the active write buffer switches to index 1, the key is
deleted in index 1, and then index 0 is flushed. If the original key
is rejournaled in the btree update from the index 0 flush, the (now
deleted) key is journaled in a seq buffer ahead of the latest
version of key (which was journaled when the key was deleted in
index 1). If the fs crashes while this is still observable in the
log, recovery sees the key from the btree update after the delete
key from the write buffer insert, which is the incorrect order. This
problem is occasionally reproduced by generic/388 and generally
manifests as one or more backpointer entry inconsistencies.

To avoid this problem, never rejournal write buffered key updates to
the associated btree. Instead, use prejournaled key updates to pass
the journal seq of the write buffer insert down to the btree insert,
which updates the btree leaf pin to reflect the seq of the key.

Note that tracking the seq is required instead of just using
NOJOURNAL here because otherwise we lose protection of the write
buffer pin when the buffer is flushed, which means the key can fall
off the tail of the on-disk journal before the btree leaf is flushed
and lead to similar recovery inconsistencies.

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