btrfs: use __bio_add_page to add single a page in rbio_add_io_sector

btrfs: use __bio_add_page to add single a page in rbio_add_io_sector

The btrfs raid56 sector submission code uses bio_add_page() to add a
page to a newly created bio. bio_add_page() can fail, but the return
value is never checked.

Use __bio_add_page() as adding a single page to a newly created bio is
guaranteed to succeed.

This brings us a step closer to marking bio_add_page() as __must_check.

Reviewed-by: Damien Le Moal <damien.lemoal@opensource.wdc.com>
Signed-off-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>