selftests/powerpc: Add a test of bad (out-of-range) accesses

Userspace isn't allowed to access certain address ranges, make sure we
actually test that to at least some degree.

This would have caught the recent bug where the SLB fault handler was
incorrectly called on an out-of-range access when using the Radix MMU.
It also would have caught the bug we had in get_region_id() where we
were inserting SLB entries for bad addresses.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20190520102051.12103-1-mpe@ellerman.id.au