netfilter: nf_tables: validate chain type update if available

Parse netlink attribute containing the chain type in this update, to
bail out if this is different from the existing type.

Otherwise, it is possible to define a chain with the same name, hook and
priority but different type, which is silently ignored.

Fixes: 96518518

 ("netfilter: add nftables")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>