libct/cgroups/GetCgroupRoot: make it faster

[kolyshkin]

...by checking the default path first.

Quick benchmark shows it's about 5x faster on an idle system, and the
gain should be much more on a system doing mounts etc.

This is an alternative to #2497. I don't like it much because it adds code rather than removes it.

[kolyshkin]

CI failure (in checkpoint --lazy-pages) looks unrelated, but the cascade of errors after it failed led me to PR #2509.

[kolyshkin]

CI restarted. The failure was

303612 checkpoint --lazy-pages and restore
3037# (from function `__runc' in file tests/integration/helpers.bash, line 57,
3038#  in test file tests/integration/checkpoint.bats, line 182)
3039#   `__runc --criu "$CRIU" restore -d --work-path ./image-dir --image-path ./image-dir --lazy-pages test_busybox_restore <&60 >&51 2>&51' failed
3040# runc list (status=0):
3041# ID          PID         STATUS      BUNDLE      CREATED     OWNER
3042# runc spec (status=0):
3043# 
3044# runc state test_busybox (status=0):
3045# {
3046#   "ociVersion": "1.0.2-dev",
3047#   "id": "test_busybox",
3048#   "pid": 3993,
3049#   "status": "running",
3050#   "bundle": "/tmp/busyboxtest",
3051#   "rootfs": "/tmp/busyboxtest/rootfs",
3052#   "created": "2020-07-07T04:09:18.881675684Z",
3053#   "owner": ""
3054# }
3055# Warn  (criu/kerndat.c:869): Can't keep kdat cache on non-tempfs
3056# runc list (status=0):
3057# ID             PID         STATUS      BUNDLE             CREATED                          OWNER
3058# test_busybox   3993        running     /tmp/busyboxtest   2020-07-07T04:09:18.881675684Z   root
3059# runc kill test_busybox KILL (status=0):
3060# 
3061# runc delete test_busybox (status=0):
3062# 

[kolyshkin]

@AkihiroSuda @mrunalp PTAL