btrfs: don't allocate a btrfs_bio for raid56 per-stripe bios

Except for the spurious initialization of ->device just after allocation
nothing uses the btrfs_bio, so just allocate a normal bio without extra
data.

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