System: Fix the check which detects running out of physical memory.

System: Fix the check which detects running out of physical memory.

The code that checks whether pages allocated by allocPhysPages only checks
that the first page fits into physical memory, not that all of them do. This
change makes the code check the last page which should work properly. This
function used to only allocate one page at a time, so the first page and last
page used to be the same thing.