fib: restore JSON output for relational expressions

fib: restore JSON output for relational expressions

JSON output for the fib expression changed:

-                    "result": "check"
+                    "result": "oif"

This breaks third party JSON parsers, revert this change for relational
expressions only via workaround until there are clear rules on how to
proceed with JSON schema updates.

As for set and map statements, keep this new "check" result type since
it is not possible to peek on rhs in such case to guess if the
NFT_FIB_F_PRESENT flag needs to be set on.

Closes: https://bugzilla.netfilter.org/show_bug.cgi?id=1806
Fixes: f4b646032acf ("fib: allow to check if route exists in maps")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>