[riot-notifications] [RIOT-OS/RIOT] core/mutex: Add mutex_cancel (#15442)

[benpicco]

@benpicco commented on this pull request.

looks good to me.
Please squash, having this separated into logical commits will make the changes clearer - I think we might merge this without splitting, as the changes to `mutex_lock()` are just moving code into a helper function. 

> +              thread_getpid());
+        irq_restore(irq_state);
+    }
+    else {
+        _block(mutex, irq_state);
+    }
+}
+
+int mutex_lock_cancelable(mutex_cancel_t *mc)
+{
+    unsigned irq_state = irq_disable();
+
+    DEBUG("PID[%" PRIkernel_pid "]: mutex_lock_cancelable()\n",
+          thread_getpid());
+
+    if (mc->cancelled) {

```suggestion
    if (mc->cancelled) {
        mc->cancelled = 0;
```

so we can re-use the mutex and cancel it again

> +        irq_restore(irq_state);
+        return -ECANCELED;
+    }
+
+    mutex_t *mutex = mc->mutex;
+    if (mutex->queue.next == NULL) {
+        /* mutex is unlocked. */
+        mutex->queue.next = MUTEX_LOCKED;
+        DEBUG("PID[%" PRIkernel_pid "]: mutex_lock_cancelable() early out.\n",
+              thread_getpid());
+        irq_restore(irq_state);
+        return 0;
+    }
+    else {
+        _block(mutex, irq_state);
+        if (mc->cancelled) {

same here

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