[ros-kernel] mdl patch

[Mike Nordell]

Not really contributing to the discussion, only displaying my approval of
design.

Hartmut Birr wrote:

> - The caller is responsible for freeing a mdl for a paging io request.

This is resource ownership sense. In general, the one (or "level")
allocating/acquiring a resource is the one responsible of that resource from
there on. In user mode this has sometimes been seen as a rule-of-thumb which
can be modified for performance reasons, but in a kernel where correctness
is of utmost importance I believe this is a hard rule.

> - MmWriteToSwapPage and MmReadFromSwapPage has changed.
> The caller does only pass the physical page to this functions.

This is encapsulation sense, letting the entity owning and knowing about the
resource handling it too. Good, I like it.

/Mike