As a result of some of Al Viro's great work, here are a few cleanups

with fixes for adfs:

- factor out filename comparison, so we can be sure that adfs_compare()
  (used for namei compare) and adfs_match() (used for lookup) have the
  same behaviour.
- factor out filename lowering (which is not the same as tolower() which
  will lower top-bit-set characters) to ensure that we have the same
  behaviour when comparing filenames as when we hash them.
- factor out the object fixups, so we are applying all fixups to
  directory objects in the same way, independent of the disk format.
- factor out the object name fixup (into the previously factored out
  function) to ensure that filenames are appropriately translated -
  for example, adfs allows '/' in filenames, which being the Unix path
  separator, need to be translated to a different character, which is
  normally '.' (DOS 8.3 filenames represent the . as a / on adfs, so
  this is the expected reverse translation.)
- remove filename truncation; Al asked about this and apparently the
  decision is to remove it.  In any case, adfs's truncation was buggy,
  so this rids us of that bug by removing the truncation feature.
- we now have only one location which adds the "filetype" suffix to the
  filename, so there's no point that code being out of line.
- since we translate '/' into '.', an adfs filename of "/" or "//" would
  end up being translated to "." and ".." which have special meanings.
  In this case, change the first character to "^" to avoid these special
  directory names being abused.
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