powerpc/pseries: Disable relocation on exception while going down during crash.

Disable relocation on exception while going down even in kdump case. This
is because we are about clear htab mappings while kexec-ing into kdump
kernel and we may run into issues if we still have AIL ON.

Signed-off-by: Mahesh Salgaonkar <mahesh@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>