cgroup: don't look up existing mount point

A mount point, which is mounted by someone else, may be umounted in
any moment.

For example the test system executes tests concurrently and sometimes
one test looks up a mount point, which has been mounted by another test.

==================================== ERROR ====================================
Test: zdtm/live/static/inotify00, Namespace: 1
Dump log   : /var/lib/jenkins/jobs/CRIU-dump/workspace/test/dump/inotify00/15535/1/dump.log
--------------------------------- grep Error ---------------------------------
(00.021951) Error (cgroup.c:409): cg: failed walking /var/lib/jenkins/jobs/CRIU-dump/workspace/test/dump/signalfd00/15538/1/.criu.cgmounts.UGj28v/ for empty cgroups
(00.021967) Error (cr-dump.c:1601): Dump core (pid: 15535) failed with -1
(00.025509) Error (cr-dump.c:1914): Dumping FAILED.
------------------------------------- END -------------------------------------
================================= ERROR OVER =================================

In the previous patch I suggested to open a mount point, but it brought
other problems. We may open a directory where a cgroup mount has been
umounted and an owner will get EBUSY on attempt to remove this
directory.

Reported-by: Jenkins Criuovich
Acked-by: Tycho Andersen <tycho.andersen@canonical.com>
Signed-off-by: Andrew Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>