Revert "powerpc/64: Avoid restore_math call if possible in syscall exit"

This reverts commit bc4f65e4

.

As reported by Andreas, this commit is causing unrecoverable SLB misses in the
system call exit path:

  Unrecoverable exception 4100 at c00000000000a1ec
  Oops: Unrecoverable exception, sig: 6 [#1]
  SMP NR_CPUS=2 PowerMac
  ...
  CPU: 0 PID: 18626 Comm: rm Not tainted 4.13.0-rc3 #1
  task: c00000018335e080 task.stack: c000000139e50000
  NIP: c00000000000a1ec LR: c00000000000a118 CTR: 0000000000000000
  REGS: c000000139e53bb0 TRAP: 4100   Not tainted  (4.13.0-rc3)
  MSR: 9000000000001030 <SF,HV,ME,IR,DR> CR: 24000044  XER: 20000000 SOFTE: 1
  GPR00: 0000000000000000 c000000139e53e30 c000000000abb500 fffffffffffffffe
  GPR04: c0000001eb866298 0000000000000000 0000000000000000 c00000018335e080
  GPR08: 900000000000d032 0000000000000000 0000000000000002 fffffffffffff001
  GPR12: c000000139e50000 c00000000ffff000 00003fffa8c0dca0 00003fffa8c0dc88
  GPR16: 0000000010000000 0000000000000001 00003fffa8c0eaa0 0000000000000000
  GPR20: 00003fffa8c27528 00003fffa8c27b00 0000000000000000 0000000000000000
  GPR24: 00003fffa8c0d918 00003ffff1b3efa0 00003fffa8c26d68 0000000000000000
  GPR28: 00003fffa8c249e8 00003fffa8c263d0 00003fffa8c27550 00003ffff1b3ef10
  NIP [c00000000000a1ec] system_call_exit+0xc0/0x21c
  LR [c00000000000a118] system_call+0x58/0x6c
  Call Trace:
  [c000000139e53e30] [c00000000000a118] system_call+0x58/0x6c (unreliable)
  Instruction dump:
  64a51000 7c6300d0 f8a101a0 4bffff9c 3c000000 60000006 780007c6 64000000
  60000000 7c004039 4082001c e8ed0170 <88070b78> 88c70b79 7c003214 2c200000

This is caused by us trying to load THREAD_LOAD_FP with MSR_RI=0, and taking an
SLB miss on the thread struct.

Reported-by: Andreas Schwab <schwab@linux-m68k.org>
Diagnosed-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>