kernel - Fix int/long truncation problem in rman_reserve_resource()

* Use ulmin/ulmax instead of min/max in three places, fixing a 32-bit
  truncation problem when setting up memory resources that caused our
  48-core opteron to panic.

* An earlier commit adjusting count values to be more correct revealed
  the bug.

* Fixes booting the kernel on our 48-core opteron w/128G of ram.