LU-15451 sec: read-only nodemap flag

LU-15451 sec: read-only nodemap flag

Add a new 'readonly_mount' property to nodemaps. When set, we return
-EROFS from server side if the client is not mounting read-only.
So the client will have to specify the read-only mount option to be
allowed to mount.

Lustre-change: https://review.whamcloud.com/46149
Lustre-commit: TBD (a8cbf96538dbee8104180f63af037cbc5c9e08e9)

Fixes: 928714dddabb ("LU-5092 nodemap: save id maps to targets in new index file")
Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: I9931844ae46dfd5d724f592f8dfacc4a8011c7e3
Reviewed-on: https://review.whamcloud.com/47816
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>