tools/sched_ext: scx_flatcg: fix potential stack overflow from VLA in fcg_read_stats

tools/sched_ext: scx_flatcg: fix potential stack overflow from VLA in fcg_read_stats

fcg_read_stats() had a VLA allocating 21 * nr_cpus * 8 bytes on the
stack, risking stack overflow on large CPU counts (nr_cpus can be up
to 512).

Fix by using a single heap allocation with the correct size, reusing
it across all stat indices, and freeing it at the end.

Signed-off-by: David Carlier <devnexen@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>