sched: change timer state inside atomic context

timer_base is a thread-agnostic interface for per-CPU timers.  The
current code is prone to a race condition involving set() and
cancel(). The latter may attempt to remove the timer before it was
inserted into timer tree.

Found during code inspection.

Reviewed-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Tomasz Grabiec <tgrabiec@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>

core/sched.cc

@@ -1064,10 +1064,11 @@ void timer_base::expire()
 void timer_base::set(osv::clock::uptime::time_point time)
 {
     trace_timer_set(this, time.time_since_epoch().count());
-    _state = state::armed;
-    _time = time;
     irq_save_lock_type irq_lock;
     WITH_LOCK(irq_lock) {
+        _state = state::armed;
+        _time = time;
+
         auto& timers = cpu::current()->timers;
         timers._list.insert(*this);
         _t._active_timers.push_back(*this);