fib_rules: fix error in backport of e9919a24 ("fib_rules: return 0...")

When commit e9919a24 ("fib_rules: return 0 directly if an exactly
same rule exists when NLM_F_EXCL not supplied") was backported to 4.9.y,
it changed the logic a bit as err should have been reset before exiting
the test, like it happens in the original logic.

If this is not set, errors happen :(
Reported-by: Nathan Chancellor <natechancellor@gmail.com>
Reported-by: David Ahern <dsahern@gmail.com>
Reported-by: Florian Westphal <fw@strlen.de>
Cc: Hangbin Liu <liuhangbin@gmail.com>
Cc: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

net/core/fib_rules.c

@@ -430,6 +430,7 @@ int fib_nl_newrule(struct sk_buff *skb, struct nlmsghdr *nlh)
 		goto errout_free;
 
 	if (rule_exists(ops, frh, tb, rule)) {
+		err = 0;
 		if (nlh->nlmsg_flags & NLM_F_EXCL)
 			err = -EEXIST;
 		goto errout_free;