netfilter: nf_tables: validate registers coming from userspace.

commit 6e1acfa3 upstream.

Bail out in case userspace uses unsupported registers.

Fixes: 49499c3e ("netfilter: nf_tables: switch registers to 32 bit addressing")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>