linux: fallback to receiver-side device creation when fsopen fails

When fsopen() is not available (returns ENOSYS), the host-side device
pre-creation in prepare_and_send_dev_mounts() cannot proceed since it
relies on the new mount API to create a detached tmpfs.

Instead of failing, send empty fds to the container process so that
libcrun_create_dev() falls back to its existing legacy path (bind
mounts or mknod) inside the container's mount namespace.

Closes: #2104

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>

Author: giuseppe

src/libcrun/linux.c

@@ -5440,7 +5440,7 @@ prepare_and_send_dev_mounts (libcrun_container_t *container, int sync_socket_hos
   devs_mountfd = fsopen_mount ("tmpfs", NULL, context_type, label, NULL);
   if (UNLIKELY (devs_mountfd < 0))
     {
-      ret = crun_make_error (err, errno, "fsopen_mount `tmpfs`");
+      ret = send_mounts (sync_socket_host, dev_fds, 0, def->linux->devices_len, err);
       goto restore_mountns;
     }