powerpc/64: interrupt soft-enable race fix

Prevent interrupt restore from allowing racing hard interrupts going
ahead of previous soft-pending ones, by using the soft-masked restart
handler to allow a store to clear the soft-mask while knowing nothing
is soft-pending.

This probably doesn't matter much in practice, but it's a simple
demonstrator / test case to exercise the restart table logic.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210617155116.2167984-11-npiggin@gmail.com