net: mctp: use nlmsg_payload() for netlink message data extraction

Jakub suggests:

> I have a different request :) Matt, once this ends up in net-next
> (end of this week) could you refactor it to use nlmsg_payload() ?
> It doesn't exist in net but this is exactly why it was added.

This refactors the additions to both mctp_dump_addrinfo(), and
mctp_rtm_getneigh() - two cases where we're calling nlh_data() on an
an incoming netlink message, without a prior nlmsg_parse().

For the neigh.c case, we cannot hit the failure where the nlh does not
contain a full ndmsg at present, as the core handler
(net/core/neighbour.c, neigh_get()) has already validated the size
through neigh_valid_req_get(), and would have failed the get operation
before the MCTP hander is called.

However, relying on that is a bit fragile, so apply the nlmsg_payload
refector here too.

Reviewed-by: Simon Horman <[e-mail]>
Signed-off-by: Jeremy Kerr <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Paolo Abeni <[e-mail]>

net/mctp/device.c

···
 	int ifindex = 0, rc;
 
 	/* Filter by ifindex if a header is provided */
-	if (cb->nlh->nlmsg_len >= nlmsg_msg_size(sizeof(*hdr))) {
-		hdr = nlmsg_data(cb->nlh);
+	hdr = nlmsg_payload(cb->nlh, sizeof(*hdr));
+	if (hdr) {
 		ifindex = hdr->ifa_index;
 	} else {
 		if (cb->strict_check) {

net/mctp/neigh.c

···
 		int idx;
 	} *cbctx = (void *)cb->ctx;
 
-	ndmsg = nlmsg_data(cb->nlh);
+	ndmsg = nlmsg_payload(cb->nlh, sizeof(*ndmsg));
+	if (!ndmsg)
+		return -EINVAL;
+
 	req_ifindex = ndmsg->ndm_ifindex;
 
 	idx = 0;