dissect-image: also derive read-only mode from fstype in non-partitioned mode

For the GPT partitioned logic we also consult the fstype to determine whether
a partition is read-only (i.e. squashfs is already read-only). For the
non-partitioned mode we didn't do that so far. Fix that.

src/shared/dissect-image.c

@@ -816,7 +816,7 @@ int dissect_image(
 
                         m->partitions[PARTITION_ROOT] = (DissectedPartition) {
                                 .found = true,
-                                .rw = !m->verity_ready,
+                                .rw = !m->verity_ready && !fstype_is_ro(fstype),
                                 .partno = -1,
                                 .architecture = _ARCHITECTURE_INVALID,
                                 .fstype = TAKE_PTR(t),