netfilter: nf_tables: don't use position attribute on rule replacement

Its possible to set both HANDLE and POSITION when replacing a rule.
In this case, the rule at POSITION gets replaced using the
userspace-provided handle.  Rule handles are supposed to be generated
by the kernel only.

Duplicate handles should be harmless, however better disable this "feature"
by only checking for the POSITION attribute on insert operations.

Fixes: 5e948466 ("netfilter: nf_tables: add insert operation")
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>