zdtm: simplify a case, when a root should be changed

We don't need to umount each points one by one,
if we are goning to do pivot_root().

"""
pivot_root moves the root file system of the current process to the
directory put_old and makes new_root the new root file system.
"""

So I suggest to do pivot_root() and then detach the old root, all
other mount points will be unmounted automatically.

Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>