ke/mutex: do TTAS spin

This is better to do than `pause`-ing in a CAS loop.

src/kern/mutex.zig

@@ -31,11 +31,15 @@ pub const Mutex = struct {
 
         // Fast path: Try to acquire without a turnstile by spinning a bit.
         for (0..optimistic_spins) |_| {
-            if (m.owner.cmpxchgWeak(null, curtd, .acquire, .monotonic) != null) {
+            if (m.owner.load(.monotonic) != null) {
                 std.atomic.spinLoopHint();
                 continue;
             }
 
+            if (m.owner.cmpxchgWeak(null, curtd, .acquire, .monotonic) != null) {
+                continue;
+            }
+
             ke.ipl.lower(ipl);
             return;
         }