2.5 is 20% slower than 2.4 in an AIM9 test which is just running
readdir across /bin.  A lot of this is due to lots of tiny calls to
copy_to_user() in fs/readdir.c.  The patch speeds up that test by 50%,
so it's comfortably faster than 2.4.

Also, there were lots of unchecked copy_to_user() and put_user() calls
in there.  Fixed all that up as well.

The patch assumes that each arch has a working 64-bit put_user(), which
appears to be the case.