powerpc/kexec: Fix radix to hash kexec due to IAMR/AMOR

This patch fixes a crash seen while doing a kexec from radix mode to
hash mode. Key 0 is special in hash and used in the RPN by default, we
set the key values to 0 today. In radix mode key 0 is used to control
supervisor<->user access. In hash key 0 is used by default, so the
first instruction after the switch causes a crash on kexec.

Commit 3b10d009 ("powerpc/mm/radix: Prevent kernel execution of
user space") introduced the setting of IAMR and AMOR values to prevent
execution of user mode instructions from supervisor mode. We need to
clean up these SPR's on kexec.

Fixes: 3b10d009

 ("powerpc/mm/radix: Prevent kernel execution of user space")
Cc: stable@vger.kernel.org # v4.10+
Reported-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Balbir Singh <bsingharora@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>