sock: allocate skbs from optmem

Add sock_omalloc and sock_ofree to be able to allocate control skbs,
for instance for looping errors onto sk_error_queue.

The transmit budget (sk_wmem_alloc) is involved in transmit skb
shaping, most notably in TCP Small Queues. Using this budget for
control packets would impact transmission.

Signed-off-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/net/sock.h b/include/net/sock.h
index 393c38e..0f778d3 100644
--- a/include/net/sock.h
+++ b/include/net/sock.h
@@ -1531,6 +1531,8 @@ struct sk_buff *sock_wmalloc(struct sock *sk, unsigned long size, int force,
 			     gfp_t priority);
 void __sock_wfree(struct sk_buff *skb);
 void sock_wfree(struct sk_buff *skb);
+struct sk_buff *sock_omalloc(struct sock *sk, unsigned long size,
+			     gfp_t priority);
 void skb_orphan_partial(struct sk_buff *skb);
 void sock_rfree(struct sk_buff *skb);
 void sock_efree(struct sk_buff *skb);