powerpc/interrupt: Don't read MSR from interrupt_exit_kernel_prepare()

A disassembly of interrupt_exit_kernel_prepare() shows a useless read
of MSR register. This is shown by r9 being re-used immediately without
doing anything with the value read.

  c000e0e0:       60 00 00 00     nop
  c000e0e4:       7d 3a c2 a6     mfmd_ap r9
  c000e0e8:       7d 20 00 a6     mfmsr   r9
  c000e0ec:       7c 51 13 a6     mtspr   81,r2
  c000e0f0:       81 3f 00 84     lwz     r9,132(r31)
  c000e0f4:       71 29 80 00     andi.   r9,r9,32768

This is due to the use of local_irq_save(). The flags read by
local_irq_save() are never used, use local_irq_disable() instead.

Fixes: 13799748 ("powerpc/64: use interrupt restart table to speed up return from interrupt")
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Reviewed-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/df36c6205ab64326fb1b991993c82057e92ace2f.1685955214.git.christophe.leroy@csgroup.eu

arch/powerpc/kernel/interrupt.c

@@ -368,7 +368,6 @@ void preempt_schedule_irq(void);
 
 notrace unsigned long interrupt_exit_kernel_prepare(struct pt_regs *regs)
 {
-	unsigned long flags;
 	unsigned long ret = 0;
 	unsigned long kuap;
 	bool stack_store = read_thread_flags() & _TIF_EMULATE_STACK_STORE;
@@ -392,7 +391,7 @@ notrace unsigned long interrupt_exit_kernel_prepare(struct pt_regs *regs)
 
 	kuap = kuap_get_and_assert_locked();
 
-	local_irq_save(flags);
+	local_irq_disable();
 
 	if (!arch_irq_disabled_regs(regs)) {
 		/* Returning to a kernel context with local irqs enabled. */