Fix 2 bugs :

• A race condition could happen if two threads were using RAS at the same time

as the code didn't reset RAS_END, the RAS code could believe we were not in
a RAS, when we were in fact.

• Using signed value logic to compare addresses wasn't such a good idea.

Many thanks to Ian to investigate on these issues.

Pointy hat to: cognet
PR: arm/161498
Submitted by: Ian Lepore <freebsd At damnhippie DOT dyndns dot org
MFC after: 1 week