net: fix nla_policy_len to actually _iterate_ over the policy

Currently nla_policy_len always returns n * NLA_HDRLEN:
It loops, but does not advance it's iterator.
NLA_UNSPEC == 0 does not contain a .len in any policy.

Trivially fixed by adding p++.

Signed-off-by: Lars Ellenberg <lars.ellenberg@linbit.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/lib/nlattr.c b/lib/nlattr.c
index 5021cbc..ac09f22 100644
--- a/lib/nlattr.c
+++ b/lib/nlattr.c
@@ -148,7 +148,7 @@
 {
 	int i, len = 0;
 
-	for (i = 0; i < n; i++) {
+	for (i = 0; i < n; i++, p++) {
 		if (p->len)
 			len += nla_total_size(p->len);
 		else if (nla_attr_minlen[p->type])