powerpc/pseries/mce: restore msr before returning from handler

The pseries real-mode machine check handler can enable the MMU, and
return from the handler with the MMU still enabled.

This works, but real-mode handler wrapper exit handlers want to rely
on the MMU being in real-mode. So change the pseries handler to
restore the MSR after it has finished virtual mode tasks.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/1612702361.lm7fqo56re.astroid@bobo.none