[XFS] Now that xfs_setattr is only used for attributes set from ->setattr

it can be switched to take struct iattr directly and thus simplify the
implementation greatly. Also rename the ATTR_ flags to XFS_ATTR_ to not
conflict with the ATTR_ flags used by the VFS.

SGI-PV: 984565

SGI-Modid: xfs-linux-melb:xfs-kern:31678a

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