[Call for Tests] PAT issue on Apple hardware

Often times I hear complaints like "my Mac hangs after upgrading to 
8.1" or "snapshot CD hangs on my brand new Mac".  I know some of 
these complaints started happening when we switched to new PAT 
layout.  It is so puzzling because it never happened on non-Apple 
hardware, AFAIK.  I really like to fix this problem but I cannot 
afford a Mac. :-P

If you are one of those lucky people, please test the attached patch 
and report your hardware model and any improvement or regression.

Also, I added a new tunable "vm.pmap.pat_works" so that you can turn 
it off from loader (i.e., "set vm.pmap.pat_works=0") and restore old 
behaviour without recompiling a new kernel.

Thanks in advance!

Jung-uk Kim