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:

	If you need to map two pages because you want to copy
	from one page to another you need to keep the kmap_atomic
	calls strictly nested, like:

	vaddr1 = kmap_atomic(page1);
	vaddr2 = kmap_atomic(page2);

	memcpy(vaddr1, vaddr2, PAGE_SIZE);

	kunmap_atomic(vaddr2);
	kunmap_atomic(vaddr1);

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

Change-Id: Iae08ed39f42df7088e72acd001e67c84a926a1b1
Reported-by: Lime Yang <limey@codeaurora.org>
Signed-off-by: Vikram Mulukutla <markivx@codeaurora.org>