LU-4423 libcfs: remove IS_PO2 and __is_po2

Removes IS_PO2 and __is_po2 since the uses of IS_PO2 have
been replaced by is_power_of_2

Linux-commit: d4891039904fa25edf1ca793a0469633ed81df3f

The following commit message is the same for the following
patches:

hash.c: Replace IS_PO2 by is_power_of_2

Linux-commit: 71872e9cc2af4dca1903ebc57daa15f08c795d86

selftest.h: replace IS_PO2 by is_power_of_2

Linux-commit: b3367164f4ff8ff2c1aa8bd79c7548f113b62b83

workitem.c: replace IS_PO2 by is_power_of_2

Linux-commit: 57b573d14b0fb9f83575a2cf155862d251c8f0d1

ldlm_extent.c: replace IS_PO2 by is_power_of_2

Linux-commit: 5f4179e04b31441b0b7995d14320a457aafba01b

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: Ic8bb40394b46ea433e3096c878abe467eacc7996
Signed-off-by: Aya Mahfouz <mahfouz.saif.elyazal@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: James Simmons <uja.ornl@yahoo.com>
Reviewed-on: https://review.whamcloud.com/24577
Tested-by: Jenkins
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Jian Yu <jian.yu@intel.com>