btrfs: raid56: extract the pq generation code into a helper

Currently finish_rmw() will update the P/Q stripes before submitting
the writes.

It's done behind a for(;;) loop, it's a little congested indent-wise, so
extract the code into a helper called generate_pq_vertical().

Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>