xfsprogs: push down unsigned char casts

xfsprogs: push down unsigned char casts

To get us of casting hell, push down the casts to the unsigned char
to the low-level hash code and the on-disk format definitions.  The
hash code is the only place that actually cares about the signedness.
The on-disk format would be cleaner by not using it, but using it
is a correct headsup to worry about the signedness.

Signed-off-by: Christoph Hellwig <hch@lst.de>