bcachefs: Fix deadlock in journal write path

bch2_journal_write() was incorrectly waiting on earlier journal writes
synchronously; this usually worked because most of the time we'd be
running in the context of a thread that did a journal_buf_put(), but
sometimes we'd be running out of the same workqueue that completes those
prior journal writes.

Additionally, this makes sure to punt to a workqueue before submitting
preflushes - we really don't want to be calling submit_bio() in the main
transaction commit path.

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