net: unix: Align send data_len up to PAGE_SIZE

Using whole of allocated pages reduces requested skb->data size.
This is just a little more thriftily allocation.

netperf does not show difference with the current performance.

Signed-off-by: Kirill Tkhai <ktkhai@parallels.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>