bcachefs: bch2_btree_write_buffer_flush_going_ro()

The write buffer needs to be specifically flushed when going RO: keys in
the journal that haven't yet been moved to the write buffer don't have a
journal pin yet.

This fixes numerous syzbot bugs, all with symptoms of still doing writes
after we've got RO.

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