KVM: selftests: Zero out valid_bank_mask for "all" case in Hyper-V IPI test

Zero out the valid_bank_mask when using the fast variant of
HVCALL_SEND_IPI_EX to send IPIs to all vCPUs.  KVM requires the "var_cnt"
and "valid_bank_mask" inputs to be consistent even when targeting all
vCPUs.  See commit bd1ba573 ("KVM: x86: Get the number of Hyper-V
sparse banks from the VARHEAD field").

Fixes: 99848924 ("KVM: selftests: Hyper-V PV IPI selftest")
Cc: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20221219220416.395329-1-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

tools/testing/selftests/kvm/x86_64/hyperv_ipi.c

@@ -193,8 +193,9 @@ static void sender_guest_code(void *hcall_page, vm_vaddr_t pgs_gpa)
 	GUEST_SYNC(stage++);
 	/*
 	 * 'XMM Fast' HvCallSendSyntheticClusterIpiEx to HV_GENERIC_SET_ALL.
-	 * Nothing to write anything to XMM regs.
 	 */
+	ipi_ex->vp_set.valid_bank_mask = 0;
+	hyperv_write_xmm_input(&ipi_ex->vp_set.valid_bank_mask, 2);
 	hyperv_hypercall(HVCALL_SEND_IPI_EX | HV_HYPERCALL_FAST_BIT,
 			 IPI_VECTOR, HV_GENERIC_SET_ALL);
 	nop_loop();