LU-8900 snapshot: simulate readonly device

LU-8900 snapshot: simulate readonly device

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.

Signed-off-by: Fan Yong <fan.yong@intel.com>
Change-Id: Iafc02bf53c41b9d7f08120ba7639b91ae527bbbc
Reviewed-on: https://review.whamcloud.com/24267
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Niu Yawei <yawei.niu@intel.com>
Reviewed-by: Lai Siyao <lai.siyao@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>