RISC-V: KVM: Synchronize hrtimer callback during teardown

In the Linux kernel, the following vulnerability has been resolved:

RISC-V: KVM: Synchronize hrtimer callback during teardown

The non-Sstc hrtimer callback clears next_set before its final uses of
the enclosing vCPU. If teardown observes next_set as false while the
callback is still running, kvm_riscv_vcpu_timer_cancel() skips
hrtimer_cancel() and kvm_destroy_vcpus() can free the vCPU before the
callback enters kvm_riscv_vcpu_set_interrupt().

A guest can arm the timer with SBI TIME and request shutdown with SBI
legacy shutdown or SRST. A VMM that honors KVM_EXIT_SYSTEM_EVENT and
destroys the VM supplies the teardown side of the race; no post-launch
host ioctl is needed to arm or request teardown.

On upstream master 62cc90241548, generic KASAN reported:

BUG: KASAN: slab-use-after-free in do_raw_spin_lock
Write of size 4 at addr ff60000005e58898

kvm_riscv_vcpu_set_interrupt
kvm_riscv_vcpu_hrtimer_expired
__hrtimer_run_queues
hrtimer_interrupt

The object was allocated by KVM_CREATE_VCPU and freed concurrently by:

kvm_destroy_vcpus
kvm_arch_destroy_vm
kvm_destroy_vm
__fput

For deterministic validation, I added mdelay(1000) immediately after
the existing next_set = false assignment. This only widens the
existing post-clear callback window. A no-delay trace build naturally
reached the callback-after-teardown-start/before-deinit ordering in 12
of 200 runs, but 1,500 stock-kernel stress iterations did not produce a
KASAN report, so natural reproduction is timing-sensitive.

Always invoke hrtimer_cancel() for an initialized timer. Preserve the
existing -EINVAL result when the timer is no longer set, but only after
synchronizing with a running callback.

With this patch, hrtimer_cancel() blocked for the full widened callback
window before vCPU destruction. KASAN reported no error in 100
fixed-and-widened runs or 200 fix-only timing-sweep runs.