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>

include/uapi/linux/packet_diag.h

@@ -33,9 +33,11 @@ enum {
 	PACKET_DIAG_TX_RING,
 	PACKET_DIAG_FANOUT,
 
-	PACKET_DIAG_MAX,
+	__PACKET_DIAG_MAX,
 };
 
+#define PACKET_DIAG_MAX (__PACKET_DIAG_MAX - 1)
+
 struct packet_diag_info {
 	__u32	pdi_index;
 	__u32	pdi_version;