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 <tgraf@suug.ch>
Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Acked-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>