netlink: revert broken, broken "2-clause nla_ok()"

Commit 4f7df337


"netlink: 2-clause nla_ok()" is BROKEN.

First clause tests if "->nla_len" could even be accessed at all,
it can not possibly be omitted.

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>