mount: Introduce @mntinfo_root variable

This variable should be carrying root of a parsed
mount tree pointed by @mntinfo. Thus if @mntinfo
get destroyed the @mntinfo_root should be set to NULL.

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