hlist_del_rcu — deletes entry from hash list without re-initialization
the element to delete from the hash list.
list_unhashed on entry does not return true after this, the entry is in an undefined state. It is useful for RCU based lockfree traversal.
In particular, it means that we can not poison the forward pointers that may still be used for walking the hash list.
The caller must take whatever precautions are necessary (such as holding appropriate locks) to avoid racing with another list-mutation primitive, such as hlist_add_head_rcu or hlist_del_rcu, running on this same list. However, it is perfectly legal to run concurrently with the _rcu list-traversal primitives, such as hlist_for_each_entry.