tipc: fix memory leak in tipc_link_xmit

In case the backlog transmit queue for system-importance messages is overloaded,
tipc_link_xmit() returns -ENOBUFS but the skb list is not purged. This leads to
memory leak and failure when a skb is allocated.

This commit fixes this issue by purging the skb list before tipc_link_xmit()
returns.

Fixes: 365ad35 ("tipc: reduce risk of user starvation during link congestion")
Signed-off-by: Tung Nguyen <tung.quang.nguyen@est.tech>
Link: https://patch.msgid.link/20250403092431.514063-1-tung.quang.nguyen@est.tech
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

net/tipc/link.c

@@ -1046,6 +1046,7 @@ int tipc_link_xmit(struct tipc_link *l, struct sk_buff_head *list,
 	if (unlikely(l->backlog[imp].len >= l->backlog[imp].limit)) {
 		if (imp == TIPC_SYSTEM_IMPORTANCE) {
 			pr_warn("%s<%s>, link overflow", link_rst_msg, l->name);
+			__skb_queue_purge(list);
 			return -ENOBUFS;
 		}
 		rc = link_schedule_user(l, hdr);