[XFS] stop using uio in the readlink code

Simplify the readlink code to get rid of the last user of uio.

SGI-PV: 968563
SGI-Modid: xfs-linux-melb:xfs-kern:29479a

Signed-off-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: David Chinner <dgc@sgi.com>
Signed-off-by: Tim Shimmin <tes@sgi.com>