We cannot assume that the mount source name always starts with 'cgroup'
so we check the filesystem type instead.

Use 'awk' instead of 'grep -E' and  as it is a better tool for this job.

This fixes the tool on systems using openrc.
Signed-off-by: Natanael Copa <ncopa@alpinelinux.org>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>