netfilter: nf_tables: perform type checking for existing sets

If a ruleset declares a set name that matches an existing set in the
kernel, then validate that this declaration really refers to the same
set, otherwise bail out with EEXIST.

Currently, the kernel reports success when adding a set that already
exists in the kernel. This usually results in EINVAL errors at a later
stage, when the user adds elements to the set, if the set declaration
mismatches the existing set representation in the kernel.

Add a new function to check that the set declaration really refers to
the same existing set in the kernel.

Fixes: 96518518

 ("netfilter: add nftables")
Reported-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>