net: fix *_DIAG_MAX constants

Follow the common pattern and define *_DIAG_MAX like:

        [...]
        __XXX_DIAG_MAX,
};

Because everyone is used to do:

        struct nlattr *attrs[XXX_DIAG_MAX+1];

        nla_parse([...], XXX_DIAG_MAX, [...]

Reported-by: Thomas Graf <tgraf@suug.ch>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Pavel Emelyanov <xemul@parallels.com>
Cc: Eric Dumazet <edumazet@google.com>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: David Howells <dhowells@redhat.com>
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>