btrfs: factor out low-level bio setup from submit_stripe_bio

Split out a low-level btrfs_submit_dev_bio helper that just submits
the bio without any cloning decisions or setting up the end I/O handler
for future reuse by a different caller.

Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>