netlink: add nla_policy_len()

It calculates the max. length of a Netlink policy, which is usefull
for allocating Netlink buffers roughly the size of the actual
message.

Signed-off-by: Holger Eitzenberger <holger@eitzenberger.org>
Signed-off-by: Patrick McHardy <kaber@trash.net>
diff --git a/include/net/netlink.h b/include/net/netlink.h
index 8a6150a..eddb502 100644
--- a/include/net/netlink.h
+++ b/include/net/netlink.h
@@ -230,6 +230,7 @@
 extern int		nla_parse(struct nlattr *tb[], int maxtype,
 				  struct nlattr *head, int len,
 				  const struct nla_policy *policy);
+extern int		nla_policy_len(const struct nla_policy *, int);
 extern struct nlattr *	nla_find(struct nlattr *head, int len, int attrtype);
 extern size_t		nla_strlcpy(char *dst, const struct nlattr *nla,
 				    size_t dstsize);