bcachefs: don't call bch2_bucket_seq_cleanup from journal_buf_switch

bcachefs: don't call bch2_bucket_seq_cleanup from journal_buf_switch

journal_buf_switch is called from the foreground when getting a journal
reservation and thus is somewhat latency sensitive;
bch2_bucket_seq_cleanup has to run infrequently but is a bit expensive
when it does run.

Call it from the journal write path instead, and punt the journal write
to worqueue context.

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