staging/lustre: replace num_physpages with totalram_pages

The global variable num_physpages is going away. Replace it
with totalram_pages.

Cc: Jiang Liu <jiang.liu@huawei.com>
Signed-off-by: Peng Tao <tao.peng@emc.com>
Signed-off-by: Andreas Dilger <andreas.dilger@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>