net: sock: fix TCP_SKB_MIN_TRUESIZE

commit eea86af6 ("net: sock: adapt SOCK_MIN_RCVBUF and
SOCK_MIN_SNDBUF") forgot the sk_buff alignment taken into account
in __alloc_skb() : skb->truesize = SKB_TRUESIZE(size);

While above commit fixed the sender issue, the receiver is still
dropping the second packet (on loopback device), because the receiver
socket can not really hold two skbs :
First packet truesize already is above sk_rcvbuf, so even TCP coalescing
cannot help.

On a typical 64bit build, each tcp skb truesize is 2304, instead of 2272
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Daniel Borkmann <dborkman@redhat.com>
Cc: Neal Cardwell <ncardwell@google.com>
Acked-by: Neal Cardwell <ncardwell@google.com>
Tested-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/sock.h

@@ -2052,7 +2052,7 @@ static inline void sk_wake_async(struct sock *sk, int how, int band)
  * Note: for send buffers, TCP works better if we can build two skbs at
  * minimum.
  */
-#define TCP_SKB_MIN_TRUESIZE	(2048 + sizeof(struct sk_buff))
+#define TCP_SKB_MIN_TRUESIZE	(2048 + SKB_DATA_ALIGN(sizeof(struct sk_buff)))
 
 #define SOCK_MIN_SNDBUF		(TCP_SKB_MIN_TRUESIZE * 2)
 #define SOCK_MIN_RCVBUF		 TCP_SKB_MIN_TRUESIZE