criu: add the mnt_id feature if a test uses more than one mntns

Signed-off-by: Andrew Vagin <avagin@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

test/zdtm/live/static/inotify01.desc

-{'flags': 'suid', 'flavor': 'ns uns'}
+{'flavor': 'ns uns', 'flags': 'suid', 'feature': 'mnt_id'}

test/zdtm/live/static/mountpoints.desc

-{'flavor': 'ns', 'flags': 'suid excl'}
+{'flavor': 'ns', 'flags': 'suid excl', 'feature': 'mnt_id'}

test/zdtm/live/static/tempfs_subns.desc

-{'flavor': 'ns uns', 'flags': 'suid'}
+{'flavor': 'ns uns', 'flags': 'suid', 'feature': 'mnt_id'}