powerpc: Rearrange SLB preload code

With the new top down layout it is likely that the pc and stack will be in the
same segment, because the pc is most likely in a library allocated via a top
down mmap. Right now we bail out early if these segments match.

Rearrange the SLB preload code to sanity check all SLB preload addresses
are not in the kernel, then check all addresses for conflicts.

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>