[XFS] Use the generic bitops rather than implementing them ourselves.

Patch inspired by Andi Kleen.

SGI-PV: 971186
SGI-Modid: xfs-linux-melb:xfs-kern:30000a

Signed-off-by: David Chinner <dgc@sgi.com>
Signed-off-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>