fib_rules: fix error return code

Fix to return error code -EINVAL from the error handling
case instead of 0, as done elsewhere in this function.

Fixes: 622ec2c9 ("net: core: add UID to flows, rules, and routes")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>