LU-6245 libcfs: replace IS_PO2 with is_power_of_2 in server code

LU-6245 libcfs: replace IS_PO2 with is_power_of_2 in server code

Replaces IS_PO2 by is_power_of_2. It is more accurate to use
is_power_of_2 since it returns 1 for numbers that are powers
of 2 only whereas IS_PO2 returns 1 for 0 and numbers that are
powers of 2.

Change-Id: I595053a658a96818ac9b434377c275d3ed7143ec
Signed-off-by: James Simmons <uja.ornl@yahoo.com>
Reviewed-on: https://review.whamcloud.com/24575
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: John L. Hammond <john.hammond@intel.com>