sock_diag: align nlattr properly when needed

I also fix the value of INET_DIAG_MAX. It's wrong since commit 8f840e47
which is only in net-next right now, thus I didn't make a separate patch.

Fixes: 8f840e47 ("sctp: add the sctp_diag.c file")
Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: David S. Miller <davem@davemloft.net>