xfs: convert attr to use unsigned names

To be consistent with the directory code, the attr code should use
unsigned names. Convert the names from the vfs at the highest level
to unsigned, and ænsure they are consistenly used as unsigned down
to disk.

Signed-off-by: Dave Chinner <david@fromorbit.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>