powerpc/64s: Fix data interrupts vs d-side MCE reentrancy

Handlers for interrupts that set DAR / DSISR, set MSR[RI] before those
SPRs are read. If a d-side machine check hits in this window, DAR /
DSISR will be clobbered silently, leading to random corruption.

Fix this by having handlers save those registers before setting MSR[RI].

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>