kcm: Destroy mutex in kcm_exit_net()

kcm_exit_net() should call mutex_destroy() on knet->mutex. This is especially
needed if CONFIG_DEBUG_MUTEXES is enabled.

Fixes: ab7ac4eb ("kcm: Kernel Connection Multiplexor module")
Signed-off-by: Shigeru Yoshida <syoshida@redhat.com>
Link: https://lore.kernel.org/r/20230902170708.1727999-1-syoshida@redhat.comSigned-off-by: Paolo Abeni <pabeni@redhat.com>

net/kcm/kcmsock.c

@@ -1859,6 +1859,8 @@ static __net_exit void kcm_exit_net(struct net *net)
 	 * that all multiplexors and psocks have been destroyed.
 	 */
 	WARN_ON(!list_empty(&knet->mux_list));
+
+	mutex_destroy(&knet->mutex);
 }
 
 static struct pernet_operations kcm_net_ops = {