ethtool: fix string set id check

Syzbot reported a shift of a u32 by more than 31 in strset_parse_request()
which is undefined behavior. This is caused by range check of string set id
using variable ret (which is always 0 at this point) instead of id (string
set id from request).

Fixes: 71921690 ("ethtool: provide string sets with STRSET_GET request")
Reported-by: syzbot+96523fb438937cd01220@syzkaller.appspotmail.com
Signed-off-by: Michal Kubecek <mkubecek@suse.cz>
Link: https://lore.kernel.org/r/b54ed5c5fd972a59afea3e1badfb36d86df68799.1607952208.git.mkubecek@suse.czSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/ethtool/strset.c

@@ -182,7 +182,7 @@ static int strset_parse_request(struct ethnl_req_info *req_base,
 		ret = strset_get_id(attr, &id, extack);
 		if (ret < 0)
 			return ret;
-		if (ret >= ETH_SS_COUNT) {
+		if (id >= ETH_SS_COUNT) {
 			NL_SET_ERR_MSG_ATTR(extack, attr,
 					    "unknown string set id");
 			return -EOPNOTSUPP;