xfs: use unsigned ints for non-negative quantities in xfs_attr_remote.c

In the next few patches we're going to refactor the attr remote code so
that we can support headerless remote xattr values for storing merkle
tree blocks.  For now, let's change the code to use unsigned int to
describe quantities of bytes and blocks that cannot be negative.

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