btrfs: move the call to bio_set_dev out of submit_stripe_bio

Prepare for additional refactoring, btrfs_map_bio is direct caller of
submit_stripe_bio.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>