KVM: arm64: Allow AArch32 PSTATE.M to be restored as System mode

It appears that we don't allow a vcpu to be restored in AArch32
System mode, as we *never* included it in the list of valid modes.

Just add it to the list of allowed modes.

Fixes: 0d854a60 ("arm64: KVM: enable initialization of a 32bit vcpu")
Cc: stable@vger.kernel.org
Acked-by: Oliver Upton <oliver.upton@linux.dev>
Link: https://lore.kernel.org/r/20240524141956.1450304-3-maz@kernel.orgSigned-off-by: Marc Zyngier <maz@kernel.org>

arch/arm64/kvm/guest.c

@@ -251,6 +251,7 @@ static int set_core_reg(struct kvm_vcpu *vcpu, const struct kvm_one_reg *reg)
 		case PSR_AA32_MODE_SVC:
 		case PSR_AA32_MODE_ABT:
 		case PSR_AA32_MODE_UND:
+		case PSR_AA32_MODE_SYS:
 			if (!vcpu_el1_is_32bit(vcpu))
 				return -EINVAL;
 			break;