netlink: Make nlmsg_find_attr take a const nlmsghdr*.

This will let us use it on a nlmsghdr stored inside a netlink_callback.
Signed-off-by: Nelson Elhage <nelhage@ksplice.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/netlink.h

@@ -384,7 +384,7 @@ static inline int nlmsg_parse(const struct nlmsghdr *nlh, int hdrlen,
  *
  * Returns the first attribute which matches the specified type.
  */
-static inline struct nlattr *nlmsg_find_attr(struct nlmsghdr *nlh,
+static inline struct nlattr *nlmsg_find_attr(const struct nlmsghdr *nlh,
 					     int hdrlen, int attrtype)
 {
 	return nla_find(nlmsg_attrdata(nlh, hdrlen),