Bug in mm/thrash.c function grab_swap_token()

Following bug was uncovered by compiling with '-W' flag:

  CC      mm/thrash.o
mm/thrash.c: In function ‘grab_swap_token’:
mm/thrash.c:52: warning: comparison of unsigned expression < 0 is always false

Variable token_priority is unsigned, so decrementing first and then
checking the result does not work; fixed by reversing the test, patch
attached (compile tested only).

I am not sure if likely() makes much sense in this new situation, but
I'll let somebody else to make a decision on that.

Signed-off-by: Mika Kukkonen <mikukkon@iki.fi>
Cc: Rik van Riel <riel@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/thrash.c

@@ -48,9 +48,8 @@ void grab_swap_token(void)
 		if (current_interval < current->mm->last_interval)
 			current->mm->token_priority++;
 		else {
-			current->mm->token_priority--;
-			if (unlikely(current->mm->token_priority < 0))
-				current->mm->token_priority = 0;
+			if (likely(current->mm->token_priority > 0))
+				current->mm->token_priority--;
 		}
 		/* Check if we deserve the token */
 		if (current->mm->token_priority >