rtnetlink: Update inet6_dump_ifinfo for strict data checking

Update inet6_dump_ifinfo for strict data checking. If the flag is
set, the dump request is expected to have an ifinfomsg struct as
the header. All elements of the struct are expected to be 0 and no
attributes can be appended.

Signed-off-by: David Ahern <dsahern@gmail.com>
Acked-by: Christian Brauner <christian@brauner.io>
Signed-off-by: David S. Miller <davem@davemloft.net>