Always take th->interrupt_lock in ubf_clear

Patch 08372635f7 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.

1 files changed, 5 insertions, 7 deletions

diff --git a/thread_pthread.c b/thread_pthread.c
index 38bc134fa8..9f6707ae5d 100644
--- a/thread_pthread.c
+++ b/thread_pthread.c
@@ -1055,14 +1055,12 @@ ubf_set(rb_thread_t *th, rb_unblock_function_t *func, void *arg)
 static void
 ubf_clear(rb_thread_t *th)
 {
-    if (th->unblock.func) {
-        rb_native_mutex_lock(&th->interrupt_lock);
-        {
-            th->unblock.func = NULL;
-            th->unblock.arg  = NULL;
-        }
-        rb_native_mutex_unlock(&th->interrupt_lock);
+    rb_native_mutex_lock(&th->interrupt_lock);
+    {
+        th->unblock.func = NULL;
+        th->unblock.arg  = NULL;
     }
+    rb_native_mutex_unlock(&th->interrupt_lock);
 }
 
 static void