hfsplus: fix worst-case unicode to char conversion of file names and attributes

hfsplus: fix worst-case unicode to char conversion of file names and attributes

This is a series of 3 patches which corrects issues in HFS+ concerning the
use of non-english file names and attributes.  Names and attributes are
stored internally as UTF-16 units up to a fixed maximum size, and convert
to and from user-representation by NLS.  The code incorrectly assume that
NLS string lengths are equal to unicode lengths, which is only true for
English ascii usage.

This patch (of 3):

The HFS Plus Volume Format specification (TN1150) states that file names
are stored internally as a maximum of 255 unicode characters, as defined
by The Unicode Standard, Version 2.0 [Unicode, Inc.  ISBN 0-201-48345-9].
File names are converted by the NLS system on Linux before presented to
the user.

255 CJK characters converts to UTF-8 with 1 unicode character to up to 3
bytes, and to GB18030 with 1 unicode character to up to 4 bytes.  Thus,
trying in a UTF-8 locale to list files with names of more than 85 CJK
characters results in:

    $ ls /mnt
    ls: reading directory /mnt: File name too long

The receiving buffer to hfsplus_uni2asc() needs to be 255 x
NLS_MAX_CHARSET_SIZE bytes, not 255 bytes as the code has always been.

Similar consideration applies to attributes, which are stored internally
as a maximum of 127 UTF-16BE units.  See XNU source for an up-to-date
reference on attributes.

Strictly speaking, the maximum value of NLS_MAX_CHARSET_SIZE = 6 is not
attainable in the case of conversion to UTF-8, as going beyond 3 bytes
requires the use of surrogate pairs, i.e.  consuming two input units.

Thanks Anton Altaparmakov for reviewing an earlier version of this change.

This patch fixes all callers of hfsplus_uni2asc(), and also enables the
use of long non-English file names in HFS+.  The getting and setting, and
general usage of long non-English attributes requires further forthcoming
work, in the following patches of this series.

Signed-off-by: Hin-Tak Leung <htl10@users.sourceforge.net>
Reviewed-by: Anton Altaparmakov <anton@tuxera.com>
Cc: Vyacheslav Dubeyko <slava@dubeyko.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: Sougata Santra <sougata@tuxera.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>