powerpc/64s/interrupt: preserve regs->softe for NMI interrupts

If an NMI interrupt hits in an implicit soft-masked region, regs->softe
is modified to reflect that. This may not be necessary for correctness
at the moment, but it is less surprising and it's unhelpful when
debugging or adding checks.

Make sure this is changed back to how it was found before returning.

Fixes: 4ec5feec ("powerpc/64s: Make NMI record implicitly soft-masked code as irqs disabled")
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210630074621.2109197-6-npiggin@gmail.com

arch/powerpc/include/asm/interrupt.h

@@ -227,6 +227,7 @@ struct interrupt_nmi_state {
 	u8 irq_soft_mask;
 	u8 irq_happened;
 	u8 ftrace_enabled;
+	u64 softe;
 #endif
 };
 
@@ -252,6 +253,7 @@ static inline void interrupt_nmi_enter_prepare(struct pt_regs *regs, struct inte
 #ifdef CONFIG_PPC64
 	state->irq_soft_mask = local_paca->irq_soft_mask;
 	state->irq_happened = local_paca->irq_happened;
+	state->softe = regs->softe;
 
 	/*
 	 * Set IRQS_ALL_DISABLED unconditionally so irqs_disabled() does
@@ -311,6 +313,7 @@ static inline void interrupt_nmi_exit_prepare(struct pt_regs *regs, struct inter
 
 	/* Check we didn't change the pending interrupt mask. */
 	WARN_ON_ONCE((state->irq_happened | PACA_IRQ_HARD_DIS) != local_paca->irq_happened);
+	regs->softe = state->softe;
 	local_paca->irq_happened = state->irq_happened;
 	local_paca->irq_soft_mask = state->irq_soft_mask;
 #endif