json: fix parse of flagcmp expression

json: fix parse of flagcmp expression

The json test case for the flagcmp notation ('tcp flags syn,fin / syn,fin') fails with:
command: {"nftables": [{"add": {"rule": {"family": "ip", "table": "test-ip4", "chain": "input", "expr": [{"match": {"left": {"&": [{"payload": {"field": "flags", "protocol": "tcp"}}, ["fin", "syn"]]}, "op": "==", "right": ["fin", "syn"]}}]}}}]}
internal:0:0-0: Error: List expression only allowed on RHS or in statement expression.
internal:0:0-0: Error: Failed to parse RHS of binop expression.
internal:0:0-0: Error: Invalid LHS of relational.
internal:0:0-0: Error: Parsing expr array at index 0 failed.
internal:0:0-0: Error: Parsing command array at index 0 failed.

Signed-off-by: Florian Westphal <fw@strlen.de>