KVM: riscv: selftests: Do not start the counter in the overflow handler

KVM: riscv: selftests: Do not start the counter in the overflow handler

There is no need to start the counter in the overflow handler as we
intend to trigger precise number of LCOFI interrupts through these
tests. The overflow irq handler has already stopped the counter. As
a result, the stop call from the test function may return already
stopped error which is fine as well.

Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Signed-off-by: Atish Patra <atishp@rivosinc.com>
Link: https://lore.kernel.org/r/20250303-kvm_pmu_improve-v2-2-41d177e45929@rivosinc.com
Signed-off-by: Anup Patel <anup@brainfault.org>