net: netlink: catch attempts to send empty messages

syzbot can figure out a way to redirect a netlink message to a tap.
Sending empty skbs to devices is not valid and we end up hitting
a skb_assert_len() in __dev_queue_xmit().

Make catching these mistakes easier, assert the skb size directly
in netlink core.

Reviewed-by: Eric Dumazet <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>

Author: kuba-moo

net/netlink/af_netlink.c

@@ -1287,6 +1287,7 @@ static struct sk_buff *netlink_trim(struct sk_buff *skb, gfp_t allocation)
 {
 	int delta;
 
+	skb_assert_len(skb);
 	WARN_ON(skb->sk != NULL);
 	delta = skb->end - skb->tail;
 	if (is_vmalloc_addr(skb->head) || delta * 2 < skb->truesize)