LU-4199 libcfs: add wrapper of PAGE_SIZE

PAGE_SIZE should not be used from userspace. It is not a
constant on all architectures and new kernels do not export
it to userspace anymore.

Signed-off-by: Li Xi <lixi@ddn.com>
Signed-off-by: James Simmons <uja.ornl@gmail.com>
Change-Id: Ib66534c31c7134833bf4768d3cc8bbc067b1bab4
Reviewed-on: http://review.whamcloud.com/8877
Tested-by: Jenkins
Reviewed-by: Jinshan Xiong <jinshan.xiong@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Bob Glossman <bob.glossman@intel.com>
Reviewed-by: Dmitry Eremin <dmitry.eremin@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>