[Unionfs] unionfs mmap diff

[Shaya Potter]

this diff changes unionfs to not pass down the address operations to the
lower file, but to do them itself.

benefits:
enables kernel/proc to know that it's mapping the union file, not the
lower file (so won't see branch in /proc or d_path().

negatives:
it's slower, as it has to read the data from the lower page, copy it to
the upper page, as well as this causes the data to be held in kernel
memory twice (leading to more memory pressure).

the patch is against unionfs-1.1.3.  I haven't done major testing
against it yet, but expect to do more testing against it in the next few
weeks.

>From what I recall, a possible problem I encounted a while ago was that
if the fs was unmounted before its contents were fully synced, then they
would be lost (so therefore have to sync before unmounting).
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