mnt: Use ns_mountpoint to open a mount point

open_mountpoint helper is called when mount namespace are
already restored so we have to use local paths.

This is a continuation to patch a082f27b -- we keep mount
point id in fsnotify, thus we should use the namespace's
path for open.

https://jira.sw.ru/browse/PSBM-39957



Signed-off-by: Andrew Vagin <avagin@openvz.org>
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>