RISC-V: KVM: do not EOPNOTSUPP in set KVM_REG_RISCV_TIMER_REG

The KVM_REG_RISCV_TIMER_REG can be read via get_one_reg(). But trying to
write anything in this reg via set_one_reg() results in an EOPNOTSUPP.

Change the API to behave like cbom_block_size: instead of always
erroring out with EOPNOTSUPP, allow userspace to write the same value
(riscv_timebase) back, throwing an EINVAL if a different value is
attempted.
Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Signed-off-by: Anup Patel <anup@brainfault.org>

arch/riscv/kvm/vcpu_timer.c

@@ -218,7 +218,8 @@ int kvm_riscv_vcpu_set_reg_timer(struct kvm_vcpu *vcpu,
 
 	switch (reg_num) {
 	case KVM_REG_RISCV_TIMER_REG(frequency):
-		ret = -EOPNOTSUPP;
+		if (reg_val != riscv_timebase)
+			return -EINVAL;
 		break;
 	case KVM_REG_RISCV_TIMER_REG(time):
 		gt->time_delta = reg_val - get_cycles64();