fix race condition in file locking

The condition occurs when
- thread #1 is holding the lock
- thread #2 is waiting for it on __futexwait
- thread #1 is about to release the lock and performs a_swap
- thread #3 enters the __lockfile function and manages to grab the lock
  before thread #1 calls __wake, resetting the MAYBE_WAITERS flag
- thread #1 calls __wake
- thread #2 wakes up but goes again to __futexwait as the lock is
  held by thread #3
- thread #3 releases the lock but does not call __wake as the
  MAYBE_WAITERS flag is not set

This condition results in thread #2 not being woken up. This patch fixes
the problem by making the woken up thread ensure that the flag is
properly set before going to sleep again.

Mainainer's note: This fixes a regression introduced in commit
c21f750727515602a9e84f2a190ee8a0a2aeb2a1.

src/stdio/__lockfile.c

@@ -8,13 +8,13 @@ int __lockfile(FILE *f)
 	int owner = f->lock, tid = __pthread_self()->tid;
 	if ((owner & ~MAYBE_WAITERS) == tid)
 		return 0;
-	for (;;) {
-		owner = a_cas(&f->lock, 0, tid);
-		if (!owner) return 1;
-		if (a_cas(&f->lock, owner, owner|MAYBE_WAITERS)==owner) break;
+	owner = a_cas(&f->lock, 0, tid);
+	if (!owner) return 1;
+	while ((owner = a_cas(&f->lock, 0, tid|MAYBE_WAITERS))) {
+		if ((owner & MAYBE_WAITERS) ||
+		    a_cas(&f->lock, owner, owner|MAYBE_WAITERS)==owner)
+			__futexwait(&f->lock, owner|MAYBE_WAITERS, 1);
 	}
-	while ((owner = a_cas(&f->lock, 0, tid|MAYBE_WAITERS)))
-		__futexwait(&f->lock, owner, 1);
 	return 1;
 }