The number of free swap pages is just a variable, and it can be obtained
much cheaper by either adding a new getter, or simply extern'ing
swap_total. However, this patch opts to use the more expensive, but
existing interface - since this isn't an operation in a high performance
path.

This allows us to remove some more gpl linuxkpi and do the following in
kms-drm:
git rm linuxkpi/gplv2/include/linux/swap.h

None yet. While this replaces stubbed functions with some
implementation and in theory should be more useful simply by the nature of it
building, it should get testing.