sh: Fix up recursive fault in oops with unset TTB.

sh: Fix up recursive fault in oops with unset TTB.

commit 90eed7d87b748f9c0d11b9bad64a4c41e31b78c4 upstream.

Presently the oops code looks for the pgd either from the mm context or
the cached TTB value. There are presently cases where the TTB can be
unset or otherwise cleared by hardware, which we weren't handling,
resulting in recursive faults on the NULL pgd. In these cases we can
simply reload from swapper_pg_dir and continue on as normal.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>