[XFS] Use kernel-supplied "roundup_pow_of_two" for simplicity

SGI-PV: 971186
SGI-Modid: xfs-linux-melb:xfs-kern:30098a

Signed-off-by: Robert P. J. Day <rpjday@crashcourse.ca>
Signed-off-by: David Chinner <dgc@sgi.com>
Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>