as detected by mtrace.

mapbench now has only one real cache line of unexpected sharing:
"name": "tlbflush_req+0x0",

use virtual addresses further away from each other in mapbench,
to avoid contending for radix_node cache lines.

This code is inefficient, it doesn't guarantee to find an unmapped
area if one exists, and it only works for crange.

in terms of performance on real hardware..  need to think more.

to avoid lost tlbflush IPIs.

..allocfd might still cause bottlenecks as threads race to allocate
FD slots..

referenced includes a "tryinc" member function that returns false if
the reference count is 0, otherwise it increments the ref. count and
returns true.  As a bonus, the implementation does not use cmpxchg
loops/retries.

Conflicts:
	kernel/sysfile.cc