cgroupv1/FindCgroupMountpoint: add a fast path
cgroupv1/FindCgroupMountpoint: add a fast path

In case cgroupPath is under the default cgroup prefix, let's try to
guess the mount point by adding the subsystem name to the default
prefix, and resolving the resulting path in case it's a symlink.

In most cases, given the default cgroup setup, this trick
should result in returning the same result faster, and avoiding
/proc/self/mountinfo parsing which is relatively slow and problematic.

Be very careful with the default path, checking it is
 - a directory;
 - a mount point;
 - has cgroup fstype.

If something is not right, fall back to parsing mountinfo.

While at it, remove the obsoleted comment about mountinfo parsing.  The
comment belongs to findCgroupMountpointAndRootFromReader(), but rather
than moving it there, let's just remove it, since it does not add any
value in understanding the current code.

Signed-off-by: Kir Kolyshkin <3a017b8ddb3f9cf3e4a59978b004111bdeb97f08@gmail.com>

xiaochenshen/runc,estesp/runc,estesp/runc,mikebrow/runc,opencontainers/runc,opencontainers/runc,dqminh/runc,dqminh/runc,kolyshkin/runc,mikebrow/runc,xiaochenshen/runc,rhatdan/runc,opencontainers/runc,xiaochenshen/runc,rhatdan/runc,mikebrow/runc,estesp/runc,rhatdan/runc,dqminh/runc,dqminh/runc,rhatdan/runc,kolyshkin/runc,kolyshkin/runc,opencontainers/runc,kolyshkin/runc