libext2fs: support encoding when calculating dx hashes

libext2fs: support encoding when calculating dx hashes

fsck must be aware of the superblock encoding and the casefold directory
setting, such that it is able to correctly calculate the dentry hashes.

[ Changed to use a stack-allocated buffer instead of a calling
  calloc() with a fixed size -- TYT ]

Signed-off-by: Gabriel Krisman Bertazi <krisman@collabora.co.uk>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>