net: Fix oops from tcp_collapse() when using splice()

tcp_read_sock() can have a eat skbs without immediately advancing copied_seq.
This can cause a panic in tcp_collapse() if it is called as a result
of the recv_actor dropping the socket lock.

A userspace program that splices data from a socket to either another
socket or to a file can trigger this bug.
Signed-off-by: Steven J. Magnani <steve@digidescorp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp.c

@@ -1368,6 +1368,7 @@ int tcp_read_sock(struct sock *sk, read_descriptor_t *desc,
 		sk_eat_skb(sk, skb, 0);
 		if (!desc->count)
 			break;
+		tp->copied_seq = seq;
 	}
 	tp->copied_seq = seq;