[XFS] Use is_power_of_2 instead of open coding checks

SGI-PV: 966576
SGI-Modid: xfs-linux-melb:xfs-kern:28950a

Signed-off-by: Vignesh Babu <vignesh.babu@wipro.com>
Signed-off-by: David Chinner <dgc@sgi.com>
Signed-off-by: Tim Shimmin <tes@sgi.com>