xfs: simplify attr name setup

Replace xfs_attr_name_to_xname with a new xfs_attr_args_init helper that
sets up the basic da_args structure without using a temporary xfs_name
structure.

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