ftests/090: Enable cpuset for root.cgroup.subtree_controller

ftests/090: Enable cpuset for root.cgroup.subtree_controller

Some version of systemd doesn't enable 'cpuset' controller for the
cgroups below default hierarchy root. Enable it, before stressing '-R'
option on cgroup v2.

-----------------------------------------------------------------
Test Results:
        Run Date:                          Jul 09 10:24:25
        Passed:                                  1 test(s)
        Skipped:                                 0 test(s)
        Failed:                                  0 test(s)
-----------------------------------------------------------------
Timing Results:
        Test                             Time (sec)
        -------------------------------------------
        setup                                  0.00
        090-cgxset-recursive_flag.py           0.27
        teardown                               0.00
        -------------------------------------------
        Total Run Time                         0.27

Signed-off-by: Kamalesh Babulal <kamalesh.babulal@oracle.com>
Signed-off-by: Tom Hromatka <tom.hromatka@oracle.com>