net: fix nla_policy_len to actually _iterate_ over the policy

Currently nla_policy_len always returns n * NLA_HDRLEN:
It loops, but does not advance it's iterator.
NLA_UNSPEC == 0 does not contain a .len in any policy.

Trivially fixed by adding p++.

Signed-off-by: Lars Ellenberg <lars.ellenberg@linbit.com>
Signed-off-by: David S. Miller <davem@davemloft.net>