KVM: arm64: Fail the idreg iterator if idregs aren't initialized

Return an error to userspace if the VM's ID register values haven't been
initialized in preparation for changing the debugfs file initialization
order.

Reviewed-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20240227094115.1723330-2-oliver.upton@linux.dev
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>

arch/arm64/kvm/sys_regs.c

@@ -3431,7 +3431,8 @@ static void *idregs_debug_start(struct seq_file *s, loff_t *pos)
 	mutex_lock(&kvm->arch.config_lock);
 
 	iter = &kvm->arch.idreg_debugfs_iter;
-	if (*iter == (u8)~0) {
+	if (test_bit(KVM_ARCH_FLAG_ID_REGS_INITIALIZED, &kvm->arch.flags) &&
+	    *iter == (u8)~0) {
 		*iter = *pos;
 		if (*iter >= KVM_ARM_ID_REG_NUM)
 			iter = NULL;