[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>