page_alloc: try to put heap control structures on the target node

page_alloc: try to put heap control structures on the target node

This also neatly eliminates (in certain cases) the need to reserve a
page between nodes, as the heap control structures will never be freed
and thus can serve as the needed separator.

It also adds an additonal fallback attempting to make use of the
wasted tail of a (group of) page(s) used for the _heap array by putting the
avail array there if it fits.

Signed-off-by: Jan Beulich <jbeulich@novell.com>