diag: warn about missing first netlink attribute

The first netlink attribute (value 0) must always be defined as none/unspec.
This is correctly done in inet_diag.h, but other diag interfaces are wrong.

Because we cannot change an existing API, I add a comment to point the mistake
and avoid to propagate it in a new diag API in the future.

CC: Thomas Graf <[e-mail]>
Signed-off-by: Nicolas Dichtel <[e-mail]>
Acked-by: Thomas Graf <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

include/uapi/linux/netlink_diag.h

···
 };
 
 enum {
+	/* NETLINK_DIAG_NONE, standard nl API requires this attribute!  */
 	NETLINK_DIAG_MEMINFO,
 	NETLINK_DIAG_GROUPS,
 	NETLINK_DIAG_RX_RING,

include/uapi/linux/packet_diag.h

···
 };
 
 enum {
+	/* PACKET_DIAG_NONE, standard nl API requires this attribute!  */
 	PACKET_DIAG_INFO,
 	PACKET_DIAG_MCLIST,
 	PACKET_DIAG_RX_RING,

include/uapi/linux/unix_diag.h

···
 };
 
 enum {
+	/* UNIX_DIAG_NONE, standard nl API requires this attribute!  */
 	UNIX_DIAG_NAME,
 	UNIX_DIAG_VFS,
 	UNIX_DIAG_PEER,