sched: relax memory ordering in cpu_set::fetch_clear

Reviewed-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Pawel Dziepak <pdziepak@quarnos.org>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>

include/osv/sched.hh

@@ -93,7 +93,8 @@ public:
     cpu_set fetch_clear() {
         cpu_set ret;
         if (_mask.load(std::memory_order_relaxed)) {
-            ret._mask = _mask.exchange(0, std::memory_order_acquire);
+            ret._mask.store(_mask.exchange(0, std::memory_order_acquire),
+                            std::memory_order_relaxed);
         }
         return ret;
     }