net/neighbor: clear error in case strict check is not set

Commit 51183d2 ("net/neighbor: Update neigh_dump_info for strict
data checking") added strict checking. The err variable is not cleared,
so if we find no table to dump we will return the validation error even
if user did not want strict checking.

I think the only way to hit this is to send an buggy request, and ask
for a table which doesn't exist, so there's no point treating this
as a real fix. I only noticed it because a syzbot repro depended on it
to trigger another bug.

Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20241115003221.733593-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: kuba-moo

net/core/neighbour.c

@@ -2824,6 +2824,7 @@ static int neigh_dump_info(struct sk_buff *skb, struct netlink_callback *cb)
 	err = neigh_valid_dump_req(nlh, cb->strict_check, &filter, cb->extack);
 	if (err < 0 && cb->strict_check)
 		return err;
+	err = 0;
 
 	s_t = cb->args[0];