[XFS] clean up xfs_start_flags

xfs_start_flags can make use of is_power_of_2 to tidy up the test a little
bit.

SGI-PV: 968563
SGI-Modid: xfs-linux-melb:xfs-kern:29327a

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