slob: fix lockup in slob_free()

Don't hold SLOB lock when freeing the page. Reduces lock hold width. See
the following thread for discussion of the bug:

  http://marc.info/?l=linux-kernel&m=123709983214143&w=2

Reported-by: Ingo Molnar <mingo@elte.hu>
Acked-by: Matt Mackall <mpm@selenic.com>
Signed-off-by: Nick Piggin <npiggin@suse.de>
Signed-off-by: Pekka Enberg <penberg@cs.helsinki.fi>

mm/slob.c

@@ -393,10 +393,11 @@ static void slob_free(void *block, int size)
 		/* Go directly to page allocator. Do not pass slob allocator */
 		if (slob_page_free(sp))
 			clear_slob_page_free(sp);
+		spin_unlock_irqrestore(&slob_lock, flags);
 		clear_slob_page(sp);
 		free_slob_page(sp);
 		free_page((unsigned long)b);
-		goto out;
+		return;
 	}
 
 	if (!slob_page_free(sp)) {