bcachefs: Break up bch2_journal_write()

Split up bch2_journal_write() to simplify locking:
 - bch2_journal_write_pick_flush(), which needs j->lock
 - bch2_journal_write_prep, which operates on the journal buffer to be
   written and will need the upcoming buf_lock for synchronization with
   the btree write buffer flush path

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