mctp: handle the struct sockaddr_mctp_ext padding field

struct sockaddr_mctp_ext.__smctp_paddin0 has to be checked for being set
to zero, otherwise it cannot be utilised in the future.

Fixes: 99ce45d5 ("mctp: Implement extended addressing")
Signed-off-by: Eugene Syromiatnikov <esyr@redhat.com>
Acked-by: Jeremy Kerr <jk@codeconstruct.com.au>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>