riscv/kvm: Fix VM hang in case of timer delta being zero.

In case when VCPU is blocked due to WFI, we schedule the timer
from `kvm_riscv_vcpu_timer_blocking()` to keep timer interrupt
ticking.

But in case when delta_ns comes to be zero, we never schedule
the timer and VCPU keeps sleeping indefinitely until any activity
is done with VM console.

This is easily reproduce-able using kvmtool.
./lkvm-static run -c1 --console virtio -p "earlycon root=/dev/vda" \
         -k ./Image -d rootfs.ext4

Also, just add a print in kvm_riscv_vcpu_vstimer_expired() to
check the interrupt delivery and run `top` or similar auto-upating
cmd from guest. Within sometime one can notice that print from
timer expiry routine stops and the `top` cmd output will stop
updating.

This change fixes this by making sure we schedule the timer even
with delta_ns being zero to bring the VCPU out of sleep immediately.

Fixes: 8f5cb44b ("RISC-V: KVM: Support sstc extension")
Signed-off-by: Rajnesh Kanwal <rkanwal@rivosinc.com>
Reviewed-by: Atish Patra <atishp@rivosinc.com>
Signed-off-by: Anup Patel <anup@brainfault.org>

arch/riscv/kvm/vcpu_timer.c

@@ -147,10 +147,8 @@ static void kvm_riscv_vcpu_timer_blocking(struct kvm_vcpu *vcpu)
 		return;
 
 	delta_ns = kvm_riscv_delta_cycles2ns(t->next_cycles, gt, t);
-	if (delta_ns) {
 	hrtimer_start(&t->hrt, ktime_set(0, delta_ns), HRTIMER_MODE_REL);
 	t->next_set = true;
-	}
 }
 
 static void kvm_riscv_vcpu_timer_unblocking(struct kvm_vcpu *vcpu)