Patch 0837263 fixed a race condition on ubfs, but it's only valid if right after
a call to `ubf_clear`, we assume the ubf function cannot be in the middle of running.
This patch removes an optimization in `ubf_clear` that violates that assumption. In short,
`ubf_clear` needs to take `th->interrupt_lock` unconditionally both to avoid deadlocks and to be
able to reason about when ubfs can be run.

This should fix CI errors like https://ci.rvm.jp/results/trunk-jemalloc@ruby-sp2-noble-docker/6242153.
The error was in test_timeout.rb, which had a deadlock during VM shutdown.

```ruby
r = Ractor.new do
begin
    Timeout.timeout(0.1) { sleep }
rescue Timeout::Error
    :ok
end
end.value

assert_equal :ok, r
```

The deadlock happened during `rb_ractor_terminate_interrupt_main_thread` with 2 ractors:

1) r1 t1: UBF called with t2->interrupt_lock (ubf = ubf_waiting)
2) r2 t2: ubf cleared from previous thread_sched_wait_events_call (but no lock taken, because of optimization)
3) r2 t2: thread_sched_wait_events: acquire thread_sched_lock(t2) (caller calling native_sleep() in loop)
4) r2 t2: ubf_set: try to acquire t2->interrupt_lock [block]
5) r1 t1: try to acquire thread_sched_lock(t2) [block, deadlock]

t2 needs to block on t2->interrupt_lock in step 2 until the ubf has completed. Only then can it register a new
ubf in the next `native_sleep` iteration.