net: fib_rules: bring back rule_exists to match rule during add

After commit f9d4b0c1 ("fib_rules: move common handling of newrule
delrule msgs into fib_nl2rule"), rule_exists got replaced by rule_find
for existing rule lookup in both the add and del paths. While this
is good for the delete path, it solves a few problems but opens up
a few invalid key matches in the add path.

$ip -4 rule add table main tos 10 fwmark 1
$ip -4 rule add table main tos 10
RTNETLINK answers: File exists

The problem here is rule_find does not check if the key masks in
the new and old rule are the same and hence ends up matching a more
secific rule. Rule key masks cannot be easily compared today without
an elaborate if-else block. Its best to introduce key masks for easier
and accurate rule comparison in the future. Until then, due to fear of
regressions this patch re-introduces older loose rule_exists during add.
Also fixes both rule_exists and rule_find to cover missing attributes.

Fixes: f9d4b0c1 ("fib_rules: move common handling of newrule delrule msgs into fib_nl2rule")
Signed-off-by: Roopa Prabhu <roopa@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>