powerpc: Fix 32-bit KVM-PR lockup and host crash with MacOS guest

Commit 8792468d "powerpc: Add the ability to save FPU without
giving it up" unexpectedly removed the MSR_FE0 and MSR_FE1 bits from
the bitmask used to update the MSR of the previous thread in
__giveup_fpu() causing a KVM-PR MacOS guest to lockup and panic the
host kernel.

Leaving FE0/1 enabled means unrelated processes might receive FPEs
when they're not expecting them and crash. In particular if this
happens to init the host will then panic.

eg (transcribed):
  qemu-system-ppc[837]: unhandled signal 8 at 12cc9ce4 nip 12cc9ce4 lr 12cc9ca4 code 0
  systemd[1]: unhandled signal 8 at 202f02e0 nip 202f02e0 lr 001003d4 code 0
  Kernel panic - not syncing: Attempted to kill init! exitcode=0x0000000b

Reinstate these bits to the MSR bitmask to enable MacOS guests to run
under 32-bit KVM-PR once again without issue.

Fixes: 8792468d ("powerpc: Add the ability to save FPU without giving it up")
Cc: stable@vger.kernel.org # v4.6+
Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/kernel/process.c

@@ -176,7 +176,7 @@ static void __giveup_fpu(struct task_struct *tsk)
 
 	save_fpu(tsk);
 	msr = tsk->thread.regs->msr;
-	msr &= ~MSR_FP;
+	msr &= ~(MSR_FP|MSR_FE0|MSR_FE1);
 #ifdef CONFIG_VSX
 	if (cpu_has_feature(CPU_FTR_VSX))
 		msr &= ~MSR_VSX;