This is the latest iteration of the workaround for the Athlon/Opteron
prefetch erratum.  Sometimes the CPU would incorrectly report an
exception on prefetch.

This supercedes the previous dumb workaround of checking for AMD CPUs in
prefetch().  That one bloated the kernel by several KB and lead to lots
of unnecessary checks in hot paths. 

Also this one handles user space faults too, so the kernel can
effectively isolte the user space from caring about this errata.

Instead it handles it in the slow path of the exception handler (the
check is only done when the kernel would normally trigger seg fault or
crash anyways)

All the serious criticisms to the previous patches have been addressed. 
It checks segment bases now, handles vm86 mode and avoids deadlocks when
the prefetch exception happened inside mmap_sem.

This includes review and fixes from Jamie Lokier and Andrew Morton.
Opcode decoder based on code from Richard Brunner.