KVM: PPC: Book3S HV: radix: Fix uninitialized var build error

Old GCCs (4.6.3 at least), aren't able to follow the logic in
__kvmhv_copy_tofrom_guest_radix() and warn that old_pid is used
uninitialized:

  arch/powerpc/kvm/book3s_64_mmu_radix.c:75:3: error: 'old_pid' may be
  used uninitialized in this function

The logic is OK, we only use old_pid if quadrant == 1, and in that
case it has definitely be initialised, eg:

	if (quadrant == 1) {
		old_pid = mfspr(SPRN_PID);
	...
	if (quadrant == 1 && pid != old_pid)
		mtspr(SPRN_PID, old_pid);

Annotate it to fix the error.
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/kvm/book3s_64_mmu_radix.c

@@ -33,8 +33,8 @@ unsigned long __kvmhv_copy_tofrom_guest_radix(int lpid, int pid,
 					      gva_t eaddr, void *to, void *from,
 					      unsigned long n)
 {
+	int uninitialized_var(old_pid), old_lpid;
 	unsigned long quadrant, ret = n;
-	int old_pid, old_lpid;
 	bool is_load = !!to;
 
 	/* Can't access quadrants 1 or 2 in non-HV mode, call the HV to do it */