btrfs: raid56: assign bio in while() when using bio_list_pop

Unify the style in the file such that return value of bio_list_pop is
assigned directly in the while loop. This is in line with the rest of
the kernel.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>