[POWERPC] Fix WARN_ON when entering xmon

Whenever we enter xmon we get a WARN_ON out of the rtas code since it
thinks interrupts are still on:

Unable to handle kernel paging request for data at address 0x00000000
Faulting instruction address: 0xd000000000080008
cpu 0x3: Vector: 300 (Data Access) at [c0000000075dba00]
    pc: d000000000080008: .doit+0x8/0x40 [oopser]
    lr: c000000000077704: .sys_init_module+0x1664/0x1824
    sp: c0000000075dbc80
   msr: 9000000000009032
   dar: 0
 dsisr: 42000000
  current = 0xc000000003fa64b0
  paca    = 0xc000000000694280
    pid   = 2260, comm = insmod

------------[ cut here ]------------
Badness at arch/powerpc/kernel/entry_64.S:651
Call Trace:
[C0000000075DAE70] [C00000000000EB64] .show_stack+0x68/0x1b0 (unreliable)
[C0000000075DAF10] [C000000000216254] .report_bug+0x94/0xe8
[C0000000075DAFA0] [C00000000047B140] __kprobes_text_start+0x178/0x584
[C0000000075DB040] [C0000000000044F4] program_check_common+0xf4/0x100
Signed-off-by: Paul Mackerras <paulus@samba.org>

arch/powerpc/xmon/xmon.c

@@ -330,18 +330,17 @@ static void release_output_lock(void)
 static int xmon_core(struct pt_regs *regs, int fromipi)
 {
 	int cmd = 0;
-	unsigned long msr;
 	struct bpt *bp;
 	long recurse_jmp[JMP_BUF_LEN];
 	unsigned long offset;
+	unsigned long flags;
 #ifdef CONFIG_SMP
 	int cpu;
 	int secondary;
 	unsigned long timeout;
 #endif
 
-	msr = mfmsr();
-	mtmsr(msr & ~MSR_EE);	/* disable interrupts */
+	local_irq_save(flags);
 
 	bp = in_breakpoint_table(regs->nip, &offset);
 	if (bp != NULL) {
@@ -516,7 +515,7 @@ static int xmon_core(struct pt_regs *regs, int fromipi)
 
 	insert_cpu_bpts();
 
-	mtmsr(msr);		/* restore interrupt enable */
+	local_irq_restore(flags);
 
 	return cmd != 'X' && cmd != EOF;
 }