powerpc/8xx: Avoid testing for kernel space in ITLB Miss.

Only modules will cause ITLB Misses as we always pin
the first 8MB of kernel memory.

Signed-off-by: Joakim Tjernlund <Joakim.Tjernlund@transmode.se>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>