af_unix: Refactor unix_read_skb()

Similar to udp_read_skb(), delete the unnecessary while loop in
unix_read_skb() for readability.  Since recv_actor() cannot return a
value greater than skb->len (see sk_psock_verdict_recv()), remove the
redundant check.
Suggested-by: Cong Wang <cong.wang@bytedance.com>
Signed-off-by: Peilin Ye <peilin.ye@bytedance.com>
Link: https://lore.kernel.org/r/7009141683ad6cd3785daced3e4a80ba0eb773b5.1663909008.git.peilin.ye@bytedance.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/unix/af_unix.c

@@ -2536,12 +2536,9 @@ static int unix_dgram_recvmsg(struct socket *sock, struct msghdr *msg, size_t si
 
 static int unix_read_skb(struct sock *sk, skb_read_actor_t recv_actor)
 {
-	int copied = 0;
-
-	while (1) {
 	struct unix_sock *u = unix_sk(sk);
 	struct sk_buff *skb;
-		int used, err;
+	int err, copied;
 
 	mutex_lock(&u->iolock);
 	skb = skb_recv_datagram(sk, MSG_DONTWAIT, &err);
@@ -2549,19 +2546,8 @@ static int unix_read_skb(struct sock *sk, skb_read_actor_t recv_actor)
 	if (!skb)
 		return err;
 
-		used = recv_actor(sk, skb);
-		if (used <= 0) {
-			if (!copied)
-				copied = used;
-			kfree_skb(skb);
-			break;
-		} else if (used <= skb->len) {
-			copied += used;
-		}
-
+	copied = recv_actor(sk, skb);
 	kfree_skb(skb);
-		break;
-	}
 
 	return copied;
 }