loop-util: use auto-detect open mode for loop device setup

When callers do not explicitly request read-only mode, pass open_flags
as -1 (auto-detect) instead of hardcoding O_RDWR. This enables the
existing O_RDWR-to-O_RDONLY retry logic in loop_device_make_by_path_at()
which falls back to O_RDONLY when opening the backing device with O_RDWR
fails with EROFS or similar errors.

Previously, callers passed O_RDWR explicitly when read-only mode was not
requested, which bypassed the retry logic entirely. This meant that
inherently read-only block devices (such as CD-ROMs) would fail to open
instead of gracefully falling back to read-only mode.

Also propagate the unresolved open_flags through
loop_device_make_by_path_at() into loop_device_make_internal() instead
of resolving it to O_RDWR early. For loop_device_make_by_path_memory(),
resolve to O_RDWR immediately since memfds are always writable.

In mstack, switch from loop_device_make() to
loop_device_make_by_path_at() with a NULL path, which reopens the
O_PATH file descriptor with the appropriate access mode. This is
necessary because the backing file descriptor is opened with O_PATH,
which prevents loop_device_make_internal() from auto-detecting the
access mode via fcntl(F_GETFL).

src/shared/loop-util.c

@@ -773,9 +773,13 @@ int loop_device_make_by_path_memory(
         int r;
 
         assert(path);
-        assert(IN_SET(open_flags, O_RDWR, O_RDONLY));
+        assert(open_flags < 0 || IN_SET(open_flags, O_RDWR, O_RDONLY));
         assert(ret);
 
+        /* memfds are always writable, so default to O_RDWR when auto-detecting. */
+        if (open_flags < 0)
+                open_flags = O_RDWR;
+
         loop_flags &= ~LO_FLAGS_DIRECT_IO; /* memfds don't support O_DIRECT, hence LO_FLAGS_DIRECT_IO can't be used either */
 
         fd = open(path, O_CLOEXEC|O_NONBLOCK|O_NOCTTY|O_RDONLY);