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.

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/46149
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Oleg Drokin <green@whamcloud.com>