Patch from Hugh Dickins

In wandering through the Linus 2.6 tree preparing for changeover of
i_mmap and i_mmap_shared to Rajesh's prio tree for object-based rmap...
I noticed that pgoff in make_coherent doesn't add up (plus, I think
we need to mask out the word "don't" in the comment further down).
2.4.25 looks equally wrong.