bcachefs: Assorted journal refactoring

bcachefs: Assorted journal refactoring

Improved the way we track various state by adding j->err_seq, which
records the first journal sequence number that encountered an error
being written, and j->last_empty_seq, which records the most recent
journal entry that was completely empty.

Also, use the low bits of the journal sequence number to index the
corresponding journal_buf.

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