KVM: PPC: Book3S HV: Pass the correct trap argument to kvmhv_commence_exit

In guest_exit_cont we call kvmhv_commence_exit which expects the trap
number as the argument. However r3 doesn't contain the trap number at
this point and as a result we would be calling the function with a
spurious trap number.

Fix this by copying r12 into r3 before calling kvmhv_commence_exit as
r12 contains the trap number.

Cc: stable@vger.kernel.org # v4.1+
Fixes: eddb60fb1443
Signed-off-by: Gautham R. Shenoy <ego@linux.vnet.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>

arch/powerpc/kvm/book3s_hv_rmhandlers.S

@@ -1257,6 +1257,7 @@ mc_cont:
 	bl	kvmhv_accumulate_time
 #endif
 
+	mr 	r3, r12
 	/* Increment exit count, poke other threads to exit */
 	bl	kvmhv_commence_exit
 	nop