drbd: interval tree: make removing an "empty" interval a no-op

Trying to remove an "empty" (just initialized, or "cleared") interval
from the tree, this results in an endless loop.

As we typically protect the tree with a spinlock_irq,
the result is a hung system.

Be nice to error cleanup code paths, ignore removal of empty intervals.

Signed-off-by: Lars Ellenberg <[e-mail]>
Signed-off-by: Christoph Böhmwalder <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jens Axboe <[e-mail]>

drivers/block/drbd/drbd_interval.c

···
 void
 drbd_remove_interval(struct rb_root *root, struct drbd_interval *this)
 {
+	/* avoid endless loop */
+	if (drbd_interval_empty(this))
+		return;
+
 	rb_erase_augmented(&this->rb, root, &augment_callbacks);
 }