api: Add support for "cgroup" controller in cgroup v2

api: Add support for "cgroup" controller in cgroup v2

The "cgroup" controller has settings that the user may want to
read/write, e.g. cgroup.controllers, cgroup.subtree_control,
cgroup.procs, etc.

Add support for this controller when the cgroup v2 mount table is
parsed by creating a custom controller for the "cgroup" settings.
Note that this feature has not been added to cgroup v1 and cgroup v1
will continue to have limited access to the cgroup.* files.

Reported-by: Justin Israel <justinisrael@gmail.com>
Reviewed-by: Kamalesh Babulal <kamalesh.babulal@oracle.com>
Signed-off-by: Tom Hromatka <tom.hromatka@oracle.com>