JSON: Simplify immediate value parsing

JSON: Simplify immediate value parsing

Since an explicit "immediate" expression doesn't exist in JSON (the
values are represented as plain JSON string/integer/boolean types),
there is no need for json_parse_immediate_expr() to comply to the common
expression parser parameter format.

Apart from that:

* Drop CTX_F_RHS checks - caller assures sane context already.
* Improve error message a bit for denied JSON types.

Signed-off-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>