runtime: fix hashing of trace stacks

The call to hash the trace stack reversed the "seed" and "size"
arguments to memhash and, hence, always called memhash with a 0 size,
which dutifully returned a hash value that depended only on the number
of PCs in the stack and not their values. As a result, all stacks were
put in to a very subset of the 8,192 buckets.

Fix this by passing these arguments in the correct order.

Change-Id: I67cd29312f5615c7ffa23e205008dd72c6b8af62
Reviewed-on: https://go-review.googlesource.com/13613Reviewed-by: Dmitry Vyukov <dvyukov@google.com>

src/runtime/trace.go

@@ -624,7 +624,7 @@ func (tab *traceStackTable) put(pcs []uintptr) uint32 {
 	if len(pcs) == 0 {
 		return 0
 	}
-	hash := memhash(unsafe.Pointer(&pcs[0]), uintptr(len(pcs))*unsafe.Sizeof(pcs[0]), 0)
+	hash := memhash(unsafe.Pointer(&pcs[0]), 0, uintptr(len(pcs))*unsafe.Sizeof(pcs[0]))
 	// First, search the hashtable w/o the mutex.
 	if id := tab.find(pcs, hash); id != 0 {
 		return id