udp: relax atomic operation on sk->sk_rmem_alloc

atomic_add_return() is more expensive than atomic_add()
and seems overkill in UDP rx fast path.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Link: https://lore.kernel.org/r/20240328144032.1864988-3-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/ipv4/udp.c

@@ -1516,12 +1516,7 @@ int __udp_enqueue_schedule_skb(struct sock *sk, struct sk_buff *skb)
 	size = skb->truesize;
 	udp_set_dev_scratch(skb);
 
-	/* we drop only if the receive buf is full and the receive
-	 * queue contains some other skb
-	 */
-	rmem = atomic_add_return(size, &sk->sk_rmem_alloc);
-	if (rmem > (size + (unsigned int)sk->sk_rcvbuf))
-		goto uncharge_drop;
+	atomic_add(size, &sk->sk_rmem_alloc);
 
 	spin_lock(&list->lock);
 	err = udp_rmem_schedule(sk, size);