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>