HFS+: refactor ASCII to unicode conversion routine for later reuse

The HFS+ filesystem is case-insensitive and does automatic unicode
decomposition by default, but does not provide custom dentry operations.  This
can lead to multiple dentries being cached for lookups on a filename with
varying case and/or character (de)composition.

These patches add custom dentry hash and comparison operations for
case-sensitive and/or automatically decomposing HFS+ filesystems.  Unicode
decomposition and case-folding are performed as required to ensure equivalent
filenames are hashed to the same values and compare as equal.

This patch:

Refactor existing HFS+ ASCII to unicode string conversion routine to split out
character conversion functionality.  This will be reused by the custom dentry
hash and comparison routines.  This approach avoids unnecessary memory
allocation compared to using the string conversion routine directly in the new
functions.

[akpm@linux-foundation.org: avoid use-of-uninitialised]
Signed-off-by: Duane Griffin <duaneg@dghda.com>
Signed-off-by: Roman Zippel <zippel@linux-m68k.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>