net: convert syn_wait_lock to a spinlock

This is a low hanging fruit, as we'll get rid of syn_wait_lock eventually.

We hold syn_wait_lock for such small sections, that it makes no sense to use
a read/write lock. A spin lock is simply faster.

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

net/ipv6/inet6_connection_sock.c

@@ -124,7 +124,7 @@ struct request_sock *inet6_csk_search_req(struct sock *sk,
 	u32 hash = inet6_synq_hash(raddr, rport, lopt->hash_rnd,
 				   lopt->nr_table_entries);
 
-	write_lock(&icsk->icsk_accept_queue.syn_wait_lock);
+	spin_lock(&icsk->icsk_accept_queue.syn_wait_lock);
 	for (req = lopt->syn_table[hash]; req != NULL; req = req->dl_next) {
 		const struct inet_request_sock *ireq = inet_rsk(req);
 
@@ -138,7 +138,7 @@ struct request_sock *inet6_csk_search_req(struct sock *sk,
 			break;
 		}
 	}
-	write_unlock(&icsk->icsk_accept_queue.syn_wait_lock);
+	spin_unlock(&icsk->icsk_accept_queue.syn_wait_lock);
 
 	return req;
 }