cache corruption patch

[Mark Tinguely]

There has been several occurrences of memory corruption in ARM due to caching
issues. A couple recent examples are problems with the SATA drive and new USB
code.

This patch tracks kernel mapped pages and detects when they are shared
with existing user mapped pages and other kernel mapped pages. This patch
has gone through some refinement in the last couple days with feed back
from testers and is getting ready for being committed before the FreeBSD 8.0
freeze.

This patch should allow the removal of added cache invalidation commands to
the dma. 

The patch is located at:

	http://www.casselton.net/~tinguely/arm_pmap_unmanaged.diff

For those interested, I can give a long explanation of the patch.

--Mark Tinguely