Introduce new server-side mount option: rdonly_dev. Under the
device readonly mode, the Lustre kernel threads that may cause
system modification, such as lfsck, scrub, osp_precreate_thread,
osp_sync_thread, and so on, will be disabled when mount. And the
system modification for recovery and client connect/disconnect,
will be ignored also.
On the other hand, if the server is mounted as "rdonly_dev", then
the client must specify "-o ro" option when mount; otherwise, it
will get -EACCES.