powerpc/mce: ensure machine check handler always tests RI

A machine check that is handled must still check MSR[RI] for
recoverability of the interrupted context. Without this patch
it's possible for a handled machine check to return to a
context where it has clobbered live registers.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210130130852.2952424-25-npiggin@gmail.com