mount: restore cwd after creating a roots yard (v2)

Currently we see that a cgroup yard are not umounted
with the ENOENT error, because cwd was changed.

v2: construct a path to remove a roots yard
Signed-off-by: Andrew Vagin <avagin@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>