tools/sched_ext: scx_userland: fix data races on shared counters

tools/sched_ext: scx_userland: fix data races on shared counters

The stats thread reads nr_vruntime_enqueues, nr_vruntime_dispatches,
nr_vruntime_failed, and nr_curr_enqueued concurrently with the main
thread writing them, with no synchronization.

Use __atomic builtins with relaxed ordering for all accesses to these
counters to eliminate the data races.

Only display accuracy is affected, not scheduling correctness.

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