skbuff: re-add check for NULL skb->head in kfree_skb path

A null check is needed after all.  netlink skbs can have skb->head be
backed by vmalloc.  The netlink destructor vfree()s head, then sets it to
NULL.  We then panic in skb_release_data with a NULL dereference.

Re-add such a test.

Alternative would be to switch to kvfree to free skb->head memory
and remove the special handling in netlink destructor.

Reported-by: kernel test robot <fengguang.wu@intel.com>
Fixes: 06dc75ab06 ("net: Revert "net: add function to allocate sk_buff head without data area")
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/skbuff.c

@@ -638,6 +638,7 @@ void skb_release_head_state(struct sk_buff *skb)
 static void skb_release_all(struct sk_buff *skb)
 {
 	skb_release_head_state(skb);
+	if (likely(skb->head))
 		skb_release_data(skb);
 }