numbers. Using ptoa() will cast to vaddr_t, which might no be adequate
for architectures where sizeof(paddr_t) > sizeof(vaddr_t) (like i386 PAE).
- small fix inside AGP heuristics to avoid masking high order bits for
systems with more than 4GB.
Reviewed by bouyer@.
See also http://mail-index.netbsd.org/tech-kern/2010/02/22/msg007373.html