tcp: clear pfmemalloc on outgoing skb

Josef Bacik diagnosed following problem :

   I was seeing random disconnects while testing NBD over loopback.
   This turned out to be because NBD sets pfmemalloc on it's socket,
   however the receiving side is a user space application so does not
   have pfmemalloc set on its socket. This means that
   sk_filter_trim_cap will simply drop this packet, under the
   assumption that the other side will simply retransmit. Well we do
   retransmit, and then the packet is just dropped again for the same
   reason.

It seems the better way to address this problem is to clear pfmemalloc
in the TCP transmit path. pfmemalloc strict control really makes sense
on the receive path.
Signed-off-by: NEric Dumazet <edumazet@google.com>
Acked-by: NJosef Bacik <jbacik@fb.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/tcp_output.c

@@ -964,6 +964,13 @@ static int tcp_transmit_skb(struct sock *sk, struct sk_buff *skb, int clone_it,
 	 */
 	skb->ooo_okay = sk_wmem_alloc_get(sk) < SKB_TRUESIZE(1);
 
+	/* If we had to use memory reserve to allocate this skb,
+	 * this might cause drops if packet is looped back :
+	 * Other socket might not have SOCK_MEMALLOC.
+	 * Packets not looped back do not care about pfmemalloc.
+	 */
+	skb->pfmemalloc = 0;
+
 	skb_push(skb, tcp_header_size);
 	skb_reset_transport_header(skb);