RDMA/cm: Add a note explaining how the timewait is eventually freed

The way the cm_timewait_info is converted into a work and then freed
is very subtle and surprising, add a note clarifying the lifetime
here.

Link: https://lore.kernel.org/r/20200506074701.9775-7-leon@kernel.orgSigned-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>

drivers/infiniband/core/cm.c

@@ -1025,6 +1025,11 @@ static void cm_enter_timewait(struct cm_id_private *cm_id_priv)
 				   msecs_to_jiffies(wait_time));
 	spin_unlock_irqrestore(&cm.lock, flags);
 
+	/*
+	 * The timewait_info is converted into a work and gets freed during
+	 * cm_free_work() in cm_timewait_handler().
+	 */
+	BUILD_BUG_ON(offsetof(struct cm_timewait_info, work) != 0);
 	cm_id_priv->timewait_info = NULL;
 }