[TCP]: Fix TCP header misalignment

Indeed my previous change to alloc_pskb has made it possible
for the TCP header to be misaligned iff the MTU is not a multiple
of 4 (and less than a page).  So I suspect the optimised IPsec
MTU calculation is giving you just such an MTU :)

This patch fixes it by changing alloc_pskb to make sure that
the size is at least 32-bit aligned.  This does not cause the
problem fixed by the previous patch because max_header is always
32-bit aligned which means that in the SG/NOTSO case this will
be a no-op.

I thought about putting this in the callers but all the current
callers are from TCP.  If and when we get a non-TCP caller we
can always create a TCP wrapper for this function and move the
alignment over there.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/sock.h

@@ -1236,6 +1236,9 @@ static inline struct sk_buff *sk_stream_alloc_pskb(struct sock *sk,
 {
 	struct sk_buff *skb;
 
+	/* The TCP header must be at least 32-bit aligned.  */
+	size = ALIGN(size, 4);
+
 	skb = alloc_skb_fclone(size + sk->sk_prot->max_header, gfp);
 	if (skb) {
 		skb->truesize += mem;