Patch from Adam J. Richter <adam@yggdrasil.com> and
           Milton Miller <miltonm@bga.com>

There's some init-time code which is supposed to read a devfs directory by
expanding the bufer until the whole directory fits.  But the logic is wrong
and it only works if the whole directory fits into 512 bytes.

So fix that up, and also clean up some coding in there, and rationalise the
duplicated definition of linux_dirent64.