netlink: constify struct nlattr * arg to parsing functions

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/netlink/attr.c b/net/netlink/attr.c
index 2d106cf..c83fea7 100644
--- a/net/netlink/attr.c
+++ b/net/netlink/attr.c
@@ -233,7 +233,7 @@
  *
  * Returns the number of bytes copied.
  */
-int nla_memcpy(void *dest, struct nlattr *src, int count)
+int nla_memcpy(void *dest, const struct nlattr *src, int count)
 {
 	int minlen = min_t(int, count, nla_len(src));