powerpc/64s/exception: machine check use correct cfar for late handler

Bare metal machine checks run an "early" handler in real mode before
running the main handler which reports the event.

The main handler runs exactly as a normal interrupt handler, after the
"windup" which sets registers back as they were at interrupt entry.
CFAR does not get restored by the windup code, so that will be wrong
when the handler is run.

Restore the CFAR to the saved value before running the late handler.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20190802105709.27696-8-npiggin@gmail.com