cifs: Override unmappable incoming uids and gids

The cifs protocol has a 64bit space for uids and gids, while linux
only supports a 32bit space today.  Instead of silently truncating
64bit cifs ids, replace cifs ids that do not fit in the 32bit linux
id space with the default uid and gids for the cifs mount.

Cc: Steve French <smfrench@gmail.com>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

fs/cifs/inode.c

@@ -244,15 +244,19 @@ cifs_unix_basic_to_fattr(struct cifs_fattr *fattr, FILE_UNIX_BASIC_INFO *info,
 		break;
 	}
 
-	if (cifs_sb->mnt_cifs_flags & CIFS_MOUNT_OVERR_UID)
-		fattr->cf_uid = cifs_sb->mnt_uid;
-	else
-		fattr->cf_uid = le64_to_cpu(info->Uid);
-
-	if (cifs_sb->mnt_cifs_flags & CIFS_MOUNT_OVERR_GID)
-		fattr->cf_gid = cifs_sb->mnt_gid;
-	else
-		fattr->cf_gid = le64_to_cpu(info->Gid);
+	fattr->cf_uid = cifs_sb->mnt_uid;
+	if (!(cifs_sb->mnt_cifs_flags & CIFS_MOUNT_OVERR_UID)) {
+		u64 id = le64_to_cpu(info->Uid);
+		if (id < ((uid_t)-1))
+			fattr->cf_uid = id;
+	}
+
+	fattr->cf_gid = cifs_sb->mnt_gid;
+	if (!(cifs_sb->mnt_cifs_flags & CIFS_MOUNT_OVERR_GID)) {
+		u64 id = le64_to_cpu(info->Gid);
+		if (id < ((gid_t)-1))
+			fattr->cf_gid = id;
+	}
 
 	fattr->cf_nlink = le64_to_cpu(info->Nlinks);
 }