powerpc/64: mark emergency stacks valid to unwind

Before:

  WARNING: CPU: 0 PID: 494 at arch/powerpc/kernel/irq.c:343
  CPU: 0 PID: 494 Comm: a Tainted: G        W
  NIP:  c00000000001ed2c LR: c000000000d13190 CTR: c00000000003f910
  REGS: c0000001fffd3870 TRAP: 0700   Tainted: G        W
  MSR:  8000000000021003 <SF,ME,RI,LE>  CR: 28000488  XER: 00000000
  CFAR: c00000000001ec90 IRQMASK: 0
  GPR00: c000000000aeb12c c0000001fffd3b00 c0000000012ba300 0000000000000000
  GPR04: 0000000000000000 0000000000000000 000000010bd207c8 6b00696e74657272
  GPR08: 0000000000000000 0000000000000000 0000000000000000 efbeadde00000000
  GPR12: 0000000000000000 c0000000014a0000 0000000000000000 0000000000000000
  GPR16: 0000000000000000 0000000000000000 0000000000000000 0000000000000000
  GPR20: 0000000000000000 0000000000000000 0000000000000000 0000000000000000
  GPR24: 0000000000000000 0000000000000000 0000000000000000 000000010bd207bc
  GPR28: 0000000000000000 c00000000148a898 0000000000000000 c0000001ffff3f50
  NIP [c00000000001ed2c] arch_local_irq_restore.part.0+0xac/0x100
  LR [c000000000d13190] _raw_spin_unlock_irqrestore+0x50/0xc0
  Call Trace:
  Instruction dump:
  60000000 7d2000a6 71298000 41820068 39200002 7d210164 4bffff9c 60000000
  60000000 7d2000a6 71298000 4c820020 <0fe00000> 4e800020 60000000 60000000

After:

  WARNING: CPU: 0 PID: 499 at arch/powerpc/kernel/irq.c:343
  CPU: 0 PID: 499 Comm: a Not tainted
  NIP:  c00000000001ed2c LR: c000000000d13210 CTR: c00000000003f980
  REGS: c0000001fffd3870 TRAP: 0700   Not tainted
  MSR:  8000000000021003 <SF,ME,RI,LE>  CR: 28000488  XER: 00000000
  CFAR: c00000000001ec90 IRQMASK: 0
  GPR00: c000000000aeb1ac c0000001fffd3b00 c0000000012ba300 0000000000000000
  GPR04: 0000000000000000 0000000000000000 00000001347607c8 6b00696e74657272
  GPR08: 0000000000000000 0000000000000000 0000000000000000 efbeadde00000000
  GPR12: 0000000000000000 c0000000014a0000 0000000000000000 0000000000000000
  GPR16: 0000000000000000 0000000000000000 0000000000000000 0000000000000000
  GPR20: 0000000000000000 0000000000000000 0000000000000000 0000000000000000
  GPR24: 0000000000000000 0000000000000000 0000000000000000 00000001347607bc
  GPR28: 0000000000000000 c00000000148a898 0000000000000000 c0000001ffff3f50
  NIP [c00000000001ed2c] arch_local_irq_restore.part.0+0xac/0x100
  LR [c000000000d13210] _raw_spin_unlock_irqrestore+0x50/0xc0
  Call Trace:
  [c0000001fffd3b20] [c000000000aeb1ac] of_find_property+0x6c/0x90
  [c0000001fffd3b70] [c000000000aeb1f0] of_get_property+0x20/0x40
  [c0000001fffd3b90] [c000000000042cdc] rtas_token+0x3c/0x70
  [c0000001fffd3bb0] [c0000000000dc318] fwnmi_release_errinfo+0x28/0x70
  [c0000001fffd3c10] [c0000000000dcd8c] pseries_machine_check_realmode+0x1dc/0x540
  [c0000001fffd3cd0] [c00000000003fe04] machine_check_early+0x54/0x70
  [c0000001fffd3d00] [c000000000008384] machine_check_early_common+0x134/0x1f0
  --- interrupt: 200 at 0x1347607c8
      LR = 0x7fffafbd8328
  Instruction dump:
  60000000 7d2000a6 71298000 41820068 39200002 7d210164 4bffff9c 60000000
  60000000 7d2000a6 71298000 4c820020 <0fe00000> 4e800020 60000000 60000000

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200325104144.158362-1-npiggin@gmail.com