net: make SO_BUSY_POLL available to all users

After commit 217f6974 ("net: busy-poll: allow preemption
in sk_busy_loop()"), a thread willing to use busy polling
is not hurting other threads anymore in a non preempt kernel.

I think it is safe to remove CAP_NET_ADMIN check.
Signed-off-by: NEric Dumazet <edumazet@google.com>
Link: https://lore.kernel.org/r/20230406194634.1804691-1-edumazet@google.comSigned-off-by: NJakub Kicinski <kuba@kernel.org>

net/core/sock.c

@@ -1396,15 +1396,10 @@ int sk_setsockopt(struct sock *sk, int level, int optname,
 
 #ifdef CONFIG_NET_RX_BUSY_POLL
 	case SO_BUSY_POLL:
-		/* allow unprivileged users to decrease the value */
-		if ((val > sk->sk_ll_usec) && !sockopt_capable(CAP_NET_ADMIN))
-			ret = -EPERM;
-		else {
-			if (val < 0)
-				ret = -EINVAL;
-			else
-				WRITE_ONCE(sk->sk_ll_usec, val);
-		}
+		if (val < 0)
+			ret = -EINVAL;
+		else
+			WRITE_ONCE(sk->sk_ll_usec, val);
 		break;
 	case SO_PREFER_BUSY_POLL:
 		if (valbool && !sockopt_capable(CAP_NET_ADMIN))