btrfs: have submit_one_bio users use bio op accessors

btrfs: have submit_one_bio users use bio op accessors

This patch has btrfs's submit_one_bio users set the bio op using
bio_set_op_attrs and get the op using bio_op.

The next patches will continue to convert btrfs,
so submit_bio_hook and merge_bio_hook
related code will be modified to take only the bio. I did
not do it in this patch to try and keep it smaller.

Signed-off-by: Mike Christie <mchristi@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Hannes Reinecke <hare@suse.com>
Signed-off-by: Jens Axboe <axboe@fb.com>