ksmbd: fix subauth 0 handling in sid_to_id()

It's not obvious why subauth 0 would be excluded from translation. This
would lead to wrong results whenever a non-identity idmapping is used.

Cc: Steve French <stfrench@microsoft.com>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: Namjae Jeon <namjae.jeon@samsung.com>
Cc: Hyunchul Lee <hyc.lee@gmail.com>
Cc: Sergey Senozhatsky <senozhatsky@chromium.org>
Cc: linux-cifs@vger.kernel.org
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>

fs/ksmbd/smbacl.c

@@ -274,7 +274,7 @@ static int sid_to_id(struct user_namespace *user_ns,
 		uid_t id;
 
 		id = le32_to_cpu(psid->sub_auth[psid->num_subauth - 1]);
-		if (id > 0) {
+		if (id >= 0) {
 			uid = make_kuid(user_ns, id);
 			if (uid_valid(uid) && kuid_has_mapping(user_ns, uid)) {
 				fattr->cf_uid = uid;
@@ -286,7 +286,7 @@ static int sid_to_id(struct user_namespace *user_ns,
 		gid_t id;
 
 		id = le32_to_cpu(psid->sub_auth[psid->num_subauth - 1]);
-		if (id > 0) {
+		if (id >= 0) {
 			gid = make_kgid(user_ns, id);
 			if (gid_valid(gid) && kgid_has_mapping(user_ns, gid)) {
 				fattr->cf_gid = gid;