xfs: minor cleanups of xfs_attr3_rmt_blocks

Clean up the type signature of this function since we don't have
negative attr lengths or block counts.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Andrey Albershteyn <aalbersh@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>