btrfs: raid56: handle endio in scrub_rbio

The only caller of scrub_rbio calls rbio_orig_end_io right after it,
move it into scrub_rbio to match the other work item helpers.

Reviewed-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>