LU-3974 libcfs: replace num_physpages with totalram_pages

Currently lustre uses an exported kernel variable
num_physpages but this variable includes also non-ram
pages and is not memory hotplug support aware. Also in
kernels 3.11 and above this variable is no longer
exported. The exported variable totalram_pages handles
the the limitations of num_physpages plus its avaliable
in all Lustre supported kernels.

Signed-off-by: James Simmons <uja.ornl@gmail.com>
Change-Id: I021fbbd99b2c8e538fc0357a676834e26d690702
Reviewed-on: http://review.whamcloud.com/7726
Reviewed-by: Bob Glossman <bob.glossman@intel.com>
Reviewed-by: Dmitry Eremin <dmitry.eremin@intel.com>
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Peng Tao <bergwolf@gmail.com>