staging: lustre: libcfs.h: 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been replaced by is_power_of_2

Signed-off-by: Aya Mahfouz <mahfouz.saif.elyazal@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>