powerpc/8xx: make user addr DTLB miss the short path

User space DTLB miss represent approximatly 90% of TLB misses
so make it the shortest path.

Also remove an unneccessary double jump in FixupDAR

Before this patch, we spend 3.3 TB ticks in the handler for each
user address miss and 3.4 TB ticks for each kernel address miss
After this patch, we send 3.0 TB ticks in the handler for each
user address miss and 3.9 TB ticks for each kernel address miss
Taking into account that user misses represent 90% of the total,
this patch provides an improvement of approx. 9%

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Scott Wood <oss@buserror.net>