[XFS] Cleanup xfs_attr a bit with xfs_name and remove cred

SGI-PV: 976035
SGI-Modid: xfs-linux-melb:xfs-kern:30913a

Signed-off-by: Barry Naujok <bnaujok@sgi.com>
Signed-off-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>