tracing: Fix unmapping loop in tracing_mark_write

Commit 6edb2a8a introduced
an array map_pages that contains the addresses returned by
kmap_atomic. However, when unmapping those pages, map_pages[0]
is unmapped before map_pages[1], breaking the nesting requirement
as specified in the documentation for kmap_atomic/kunmap_atomic.

This was caught by the highmem debug code present in kunmap_atomic.
Fix the loop to do the unmapping properly.

Link: http://lkml.kernel.org/r/1418871056-6614-1-git-send-email-markivx@codeaurora.org

Cc: stable@vger.kernel.org # 3.5+
Reviewed-by: NStephen Boyd <sboyd@codeaurora.org>
Reported-by: NLime Yang <limey@codeaurora.org>
Signed-off-by: NVikram Mulukutla <markivx@codeaurora.org>
Signed-off-by: NSteven Rostedt <rostedt@goodmis.org>

kernel/trace/trace.c

@@ -4928,7 +4928,7 @@ tracing_mark_write(struct file *filp, const char __user *ubuf,
 	*fpos += written;
 
  out_unlock:
-	for (i = 0; i < nr_pages; i++){
+	for (i = nr_pages - 1; i >= 0; i--) {
 		kunmap_atomic(map_page[i]);
 		put_page(pages[i]);
 	}