parser_json: Disallow ct helper as type to map to

parser_json: Disallow ct helper as type to map to

When creating a map, users may either map dtype:dtype or dtype:object.
In the second case, only counter, quota, limit and secmark is allowed by
bison, but JSON parser wasn't as strict, allowing ct helper as well.
Remove that to avoid undefined behaviour.

Fixes: 586ad210368b7 ("libnftables: Implement JSON parser")
Signed-off-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>