This commit makes the test program try the following cases:
* mmaping 0-offset dmabuf with 0 offset
* mmaping 0-offset dmabuf with 1 page offset
* mmaping page-offset dmabuf with 0 offset
* mmaping page-offset dmabuf with 1 page offset
* mmaping device with 0 offset
* mmaping device with 1 page offset
This makes it a compile-time error to call a function that semantically
takes no parameters with a nonzero number of arguments.
Previously, such code would still compile, but risk blowing up the stack
if a compiler chose to use something other than caller-cleanup calling
conventions.
