netfilter: nf_tables: skip flowtable hooknum and priority on device updates

On device updates, the hooknum and priority attributes are not required.
This patch makes optional these two netlink attributes.

Moreover, bail out with EOPNOTSUPP if userspace tries to update the
hooknum and priority for existing flowtables.

While at this, turn EINVAL into EOPNOTSUPP in case the hooknum is not
ingress. EINVAL is reserved for missing netlink attribute / malformed
netlink messages.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>