mctp: Be explicit about struct sockaddr_mctp padding

We currently have some implicit padding in struct sockaddr_mctp. This
patch makes this padding explicit, and ensures we have consistent
layout on platforms with <32bit alignmnent.

Fixes: 60fc6398 ("mctp: Add sockaddr_mctp to uapi")
Signed-off-by: Jeremy Kerr <jk@codeconstruct.com.au>
Signed-off-by: David S. Miller <davem@davemloft.net>