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 <[email protected]>
Signed-off-by: Jeremy Kerr <[email protected]>
Link: https://patch.msgid.link/20250521-mctp-nlmsg-payload-v2-1-e85df160c405@codeconstruct.com.au
Signed-off-by: Paolo Abeni <[email protected]>

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