KVM: PPC: Book3S HV: Fix case where HDEC is treated as 32-bit on POWER9

Commit 2f27246 ("KVM: PPC: Book3S HV: Cope with host using large
decrementer mode", 2017-05-22) added code to treat the hypervisor
decrementer (HDEC) as a 64-bit value on POWER9 rather than 32-bit.
Unfortunately, that commit missed one place where HDEC is treated
as a 32-bit value.  This fixes it.

This bug should not have any user-visible consequences that I can
think of, beyond an occasional unnecessary exit to the host kernel.
If the hypervisor decrementer has gone negative, then the bottom
32 bits will be negative for about 4 seconds after that, so as
long as we get out of the guest within those 4 seconds we won't
conclude that the HDEC interrupt is spurious.

Reported-by: Suraj Jitindar Singh <sjitindarsingh@gmail.com>
Fixes: 2f27246 ("KVM: PPC: Book3S HV: Cope with host using large decrementer mode")
Signed-off-by: Paul Mackerras <paulus@ozlabs.org>

arch/powerpc/kvm/book3s_hv_rmhandlers.S

@@ -1280,7 +1280,8 @@ END_FTR_SECTION_IFSET(CPU_FTR_HAS_PPR)
 	cmpwi	r12,BOOK3S_INTERRUPT_HV_DECREMENTER
 	bne	2f
 	mfspr	r3,SPRN_HDEC
-	cmpwi	r3,0
+	EXTEND_HDEC(r3)
+	cmpdi	r3,0
 	mr	r4,r9
 	bge	fast_guest_return
 2: