netlink: split up copies in the ack construction

Clean up the use of unsafe_memcpy() by adding a flexible array
at the end of netlink message header and splitting up the header
and data copies.

Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>