tuntap: properly align skb->head before building skb

An unaligned alloc_frag->offset caused by previous allocation will
result an unaligned skb->head. This will lead unaligned
skb_shared_info and then unaligned dataref which requires to be
aligned for accessing on some architecture. Fix this by aligning
alloc_frag->offset before the frag refilling.

Fixes: 0bbd7dad34f8 ("tun: make tun_build_skb() thread safe")
Cc: Eric Dumazet <edumazet@google.com>
Cc: Willem de Bruijn <willemdebruijn.kernel@gmail.com>
Cc: Wei Wei <dotweiba@gmail.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Reported-by: Wei Wei <dotweiba@gmail.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/tun.c

@@ -1286,6 +1286,7 @@ static struct sk_buff *tun_build_skb(struct tun_struct *tun,
 	buflen += SKB_DATA_ALIGN(len + pad);
 	rcu_read_unlock();
 
+	alloc_frag->offset = ALIGN((u64)alloc_frag->offset, SMP_CACHE_BYTES);
 	if (unlikely(!skb_page_frag_refill(buflen, alloc_frag, GFP_KERNEL)))
 		return ERR_PTR(-ENOMEM);