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>
        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;
 
        UNIX_DIAG_MEMINFO,
        UNIX_DIAG_SHUTDOWN,
 
-       UNIX_DIAG_MAX,
+       __UNIX_DIAG_MAX,
 };
 
+#define UNIX_DIAG_MAX (__UNIX_DIAG_MAX - 1)
+
 struct unix_diag_vfs {
        __u32   udiag_vfs_ino;
        __u32   udiag_vfs_dev;