net/x25: Fix potential double free of skb

When alloc_skb fails in x25_queue_rx_frame it calls kfree_skb(skb) at
line 48 and returns 1 (error).
This error propagates back through the call chain:

x25_queue_rx_frame returns 1
    |
    v
x25_state3_machine receives the return value 1 and takes the else
branch at line 278, setting queued=0 and returning 0
    |
    v
x25_process_rx_frame returns queued=0
    |
    v
x25_backlog_rcv at line 452 sees queued=0 and calls kfree_skb(skb)
again

This would free the same skb twice. Looking at x25_backlog_rcv:

net/x25/x25_in.c:x25_backlog_rcv() {
    ...
    queued = x25_process_rx_frame(sk, skb);
    ...
    if (!queued)
        kfree_skb(skb);
}

Fixes: 1da177e ("Linux-2.6.12-rc2")
Signed-off-by: Martin Schiller <ms@dev.tdt.de>
Link: https://patch.msgid.link/20260331-x25_fraglen-v4-1-3e69f18464b4@dev.tdt.de
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

Author: sch-m

net/x25/x25_in.c

@@ -44,10 +44,9 @@ static int x25_queue_rx_frame(struct sock *sk, struct sk_buff *skb, int more)
 	if (x25->fraglen > 0) {	/* End of fragment */
 		int len = x25->fraglen + skb->len;
 
-		if ((skbn = alloc_skb(len, GFP_ATOMIC)) == NULL){
-			kfree_skb(skb);
+		skbn = alloc_skb(len, GFP_ATOMIC);
+		if (!skbn)
 			return 1;
-		}
 
 		skb_queue_tail(&x25->fragment_queue, skb);