correct rcu_unlock_domain()

Match rcu_lock_domain(), and remove the slightly misleading comment:
This isn't just the companion to rcu_lock_domain_by_id() (and that
latter function indeed also keeps the domain locked, not the domain
list).

No functional change, as rcu_read_{,un}lock() ignore their arguments
anyway.

Reported-by: Jann Horn <jannh@xxxxxxxxxx>
Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/include/xen/sched.h
+++ b/xen/include/xen/sched.h
@@ -584,11 +584,10 @@ int rcu_lock_remote_domain_by_id(domid_t
  */
 int rcu_lock_live_remote_domain_by_id(domid_t dom, struct domain **d);
 
-/* Finish a RCU critical region started by rcu_lock_domain_by_id(). */
 static inline void rcu_unlock_domain(struct domain *d)
 {
     if ( d != current->domain )
-        rcu_read_unlock(&domlist_read_lock);
+        rcu_read_unlock(d);
 }
 
 static inline struct domain *rcu_lock_domain(struct domain *d)