LU-10824 llite: make foreign symlinks aware of mount namespaces

LU-10824 llite: make foreign symlinks aware of mount namespaces

Currently the foreign symlink code test if mount namespace is the
same namespace related to the sysfs tree. This doesn't cover all
cases. Linux supports limiting which mounts are visible to a
process with mount namespaces. Lets add this support as well.

Change-Id: Ie87ed45b3c4439e8800c937eb27ed4931989c0f4
Signed-off-by: James Simmons <jsimmons@infradead.org>
Reviewed-on: https://review.whamcloud.com/45609
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Faccini Bruno <bruno.faccini@intel.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>