Properly unlock the policy if no change in the kernel is required.

src/libhydra/plugins/kernel_netlink/kernel_netlink_ipsec.c

@@ -2074,6 +2074,7 @@ METHOD(kernel_ipsec_t, add_policy, status_t,
 	if (!update)
 	{	/* we don't update the policy if the priority is lower than that of the
 		 * currently installed one */
+		this->mutex->unlock(this->mutex);
 		return SUCCESS;
 	}