LU-17609 sec: nodemap readonly_mount for remount

LU-17609 sec: nodemap readonly_mount for remount

The readonly_mount property on nodemaps forces read-only mount from
clients. Clients trying rw remount (via mount -o remount,rw) should
also be forced to read-only.

Also improve sanity-sec test_61 to exercise client remount.

Fixes: e7ce67de92 ("LU-15451 sec: read-only nodemap flag")
Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: I61f8141001d2ff9e832e5c93d8f5997479af98a6
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/54282
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Arshad Hussain <arshad.hussain@aeoncomputing.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>