In order to handle the case insensitivity issues of NTFS with regards to the
dcache and the dcache requiring only one dentry per directory, we deal with
dentry aliases that only differ in case in ->ntfs_lookup() while maintining
a case sensitive dcache. This means that we get the full benefit of dcache
speed when the file/directory is looked up with the same case as returned by
->ntfs_readdir() but that a lookup for any other case (or for the short file
name) will not find anything in dcache and will enter ->ntfs_lookup()
instead, where we search the directory for a fully matching file name
(including case) and if that is not found, we search for a file name that
matches with different case and if that has non-POSIX semantics we return
that. We actually do only one search (case sensitive) and keep tabs on
whether we have found a case insensitive match in the process.

To simplify matters for us, we do not treat the short vs long filenames as
two hard links but instead if the lookup matches a short filename, we
return the dentry for the corresponding long filename instead.

In ->ntfs_lookup() we distinguish three cases:

1) @dent perfectly matches (i.e. including case) a directory entry with a
   file name in the WIN32 or POSIX namespaces. In this case
   ntfs_lookup_inode_by_name() will return with name set to NULL and we
   just d_add() @dent.
2) @dent matches (not including case) a directory entry with a file name in
   the WIN32 namespace. In this case ntfs_lookup_inode_by_name() will return
   with name set to point to a kmalloc()ed ntfs_name structure containing
   the properly cased little endian Unicode name. We convert the name to the
   current NLS code page, search if a dentry with this name already exists
   and if so return that instead of @dent. The VFS will then destroy the old
   @dent and use the one we returned. If a dentry is not found, we allocate
   a new one, d_add() it, and return it as above.
3) @dent matches either perfectly or not (i.e. we don't care about case) a
   directory entry with a file name in the DOS namespace. In this case
   ntfs_lookup_inode_by_name() will return with name set to point to a
   kmalloc()ed ntfs_name structure containing the mft reference (cpu endian)
   of the inode. We use the mft reference to read the inode and to find the
   file name in the WIN32 namespace corresponding to the matched short file
   name. We then convert the name to the current NLS code page, and proceed
   searching for a dentry with this name, etc, as in case 2), above.