[XFS] Clean up use of VFS attr flags

Use the the generic VFS attr flags where appropriate instead of open
coding them to the same values.

Patch provided by Eric Sandeen.

SGI-PV: 960868
SGI-Modid: xfs-linux-melb:xfs-kern:28033a

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