[XFS] kill sys_cred

capable_cred has been unused for a while so we can kill it and sys_cred.
That also means the cred argument to xfs_setattr and xfs_change_file_space
can be removed now.

SGI-PV: 988918

SGI-Modid: xfs-linux-melb:xfs-kern:32412a

Signed-off-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Tim Shimmin <tes@sgi.com>
Signed-off-by: David Chinner <david@fromorbit.com>
Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>