KVM: arm64: Fix misuse of KVM_ARM_VCPU_POWER_OFF bit index

KVM_ARM_VCPU_POWER_OFF is as bit index, _not_ a literal bitmask.
Nonetheless, commit e3c1c0cae3 ("KVM: arm64: Relax invariance
of KVM_ARM_VCPU_POWER_OFF") started using it that way, meaning that
powering off a vCPU with the KVM_ARM_VCPU_INIT ioctl is completely
broken.

Fix it by using a shifted bit for the bitwise operations instead.

Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Fixes: e3c1c0cae3 ("KVM: arm64: Relax invariance of KVM_ARM_VCPU_POWER_OFF")
Acked-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20230622160922.1925530-1-oliver.upton@linux.dev
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>

arch/arm64/kvm/arm.c

@@ -1280,8 +1280,8 @@ static int kvm_arch_vcpu_ioctl_vcpu_init(struct kvm_vcpu *vcpu,
 	 * reflecting it in the finalized feature set, thus limiting its scope
 	 * to a single KVM_ARM_VCPU_INIT call.
 	 */
-	if (init->features[0] & KVM_ARM_VCPU_POWER_OFF) {
-		init->features[0] &= ~KVM_ARM_VCPU_POWER_OFF;
+	if (init->features[0] & BIT(KVM_ARM_VCPU_POWER_OFF)) {
+		init->features[0] &= ~BIT(KVM_ARM_VCPU_POWER_OFF);
 		power_off = true;
 	}