Anon shmem changes tracking has known bugs.
So don't use it by default. Enable it only when
CRIU_TRACK_SHMEM env variable is defined.

Now we can run transition/shmem test in CI because it works:
python test/zdtm.py run -t zdtm/transition/shmem --pre 3

Cc: Andrei Vagin <avagin@virtuozzo.com>
Cc: Pavel Emelyanov <xemul@virtuozzo.com>
Signed-off-by: Eugene Batalov <eabatalov89@gmail.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>