fs/adfs: remove truncated filename hashing

fs/adfs support for truncated filenames is broken, and there is a desire
not to support this into the future.  Let's remove the fs/adfs support
for this.

Viro says:

"FWIW, the word from Linus had been basically "kill it off" on
truncation."

That being:

"Make it so. Make the rule be that d_hash() can only change the hash
itself, rather than the subtle special case for len that we had
because of legacy reasons.."

Acked-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>