casefold is using unsuitable case mapping table

["Björn JACKE" <bjacke@SerNet.DE>]

Hi,

I started to experiment with the casefold feature of ext4 and some other
filesystems. I was hoping to get some significant performance gains for Samba
server with large directories.

It turns out though that the case insensitive feature is not usable because it
does not match the case mapping tables that other operating systems use. More
specifically, the german letter "ß" is treated as a case equivanten of "ss".

There is an equivalent of "ß" and "ss in some other scopes, also AD LDAP treats
them as an equivante. For systems that requires "lossless" case conversion
however should not treat ß and ss as equivalent. This is also why a filesystem
should never ever do that

Since 2017 there is a well-defined uppercase version of the codepoint (U+00DF)
of the "ß" letter in Unicode: U+1E9E, this could eventually be used but I
haven't seen any filesystem using that so far. This would be a possible and
lossless case equivalent, but well, that's actually another thing to discuss.

The important point is to _not_ use the ß/ss case equicalent. The casefold
feature is mainly useless otherwise.

Can this be changed without causing too much hassle?

Cheers
Björn