powerpc/64s: masked_interrupt() returns to kernel so avoid restoring r13

Places in the kernel where r13 is not the PACA pointer must have
maskable interrupts disabled, so r13 does not have to be restored when
returning from a soft-masked interrupt. We should never have
interrupts soft disabled when we're in user space.

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