Add function prototype for NUMA-style allocation

In Linux, there is an allocator for pages which allows one
to specify the node. This is a good idea. We're not ready
to do it, but we should reserve a placeholder for it and,
when we bring code over, make sure that we don't lost track
of code that wants NUMA-specific allocation.

Add get_cont_pages_node, which takes node as the first parameter.
The function is equivalent to get_cont_pages, but that will
change if we ever get real NUMA-style allocation.

Signed-off-by: Ronald G. Minnich <rminnich@google.com>