LU-15873 obd: skip checking read-only fs health

LU-15873 obd: skip checking read-only fs health

Health check upon read-only file system would fail and STONITH
ensues.

Add obd_device::obd_read_only to record read-only flag of the
obd_device. And skip checking the health of read-only devices.

Signed-off-by: Bobi Jam <bobijam@whamcloud.com>
Change-Id: Ica83b9c871f7bee62cef6504deb0dcc32dd20afb
Reviewed-by: Wang Shilong <wshilong@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Li Xi <lixi@ddn.com>
Reviewed-on: https://review.whamcloud.com/48095
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Arshad Hussain <arshad.hussain@aeoncomputing.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>