IB/srpt: Simplify srpt_handle_tsk_mgmt()

Let the target core check task existence instead of the SRP target
driver. Additionally, let the target core check the validity of the
task management request instead of the ib_srpt driver.

This patch fixes the following kernel crash:

BUG: unable to handle kernel NULL pointer dereference at 0000000000000001
IP: [<ffffffffa0565f37>] srpt_handle_new_iu+0x6d7/0x790 [ib_srpt]
Oops: 0002 [#1] SMP
Call Trace:
 [<ffffffffa05660ce>] srpt_process_completion+0xde/0x570 [ib_srpt]
 [<ffffffffa056669f>] srpt_compl_thread+0x13f/0x160 [ib_srpt]
 [<ffffffff8109726f>] kthread+0xcf/0xe0
 [<ffffffff81613cfc>] ret_from_fork+0x7c/0xb0

Signed-off-by: Bart Van Assche <bart.vanassche@sandisk.com>
Fixes: 3e4f574857ee ("ib_srpt: Convert TMR path to target_submit_tmr")
Tested-by: Alex Estrin <alex.estrin@intel.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Cc: Nicholas Bellinger <nab@linux-iscsi.org>
Cc: Sagi Grimberg <sagig@mellanox.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Doug Ledford <dledford@redhat.com>
static void srpt_event_handler(struct ib_event_handler *handler,
			       struct ib_event *event)
{
	struct srpt_device *sdev;
	struct srpt_port *sport;

	sdev = ib_get_client_data(event->device, &srpt_client);
	if (!sdev || sdev->device != event->device)
		return;

	pr_debug("ASYNC event= %d on device= %s\n", event->event,
		 srpt_sdev_name(sdev));

	switch (event->event) {
	case IB_EVENT_PORT_ERR:
		if (event->element.port_num <= sdev->device->phys_port_cnt) {
			sport = &sdev->port[event->element.port_num - 1];
			sport->lid = 0;
			sport->sm_lid = 0;
		}
		break;
	case IB_EVENT_PORT_ACTIVE:
	case IB_EVENT_LID_CHANGE:
	case IB_EVENT_PKEY_CHANGE:
	case IB_EVENT_SM_CHANGE:
	case IB_EVENT_CLIENT_REREGISTER:
	case IB_EVENT_GID_CHANGE:
		/* Refresh port data asynchronously. */
		if (event->element.port_num <= sdev->device->phys_port_cnt) {
			sport = &sdev->port[event->element.port_num - 1];
			if (!sport->lid && !sport->sm_lid)
				schedule_work(&sport->work);
		}
		break;
	default:
		pr_err("received unrecognized IB event %d\n",
		       event->event);
		break;
	}
}

static void srpt_event_handler(struct ib_event_handler *handler,
			       struct ib_event *event)
{
	struct srpt_device *sdev;
	struct srpt_port *sport;

	sdev = ib_get_client_data(event->device, &srpt_client);
	if (!sdev || sdev->device != event->device)
		return;

	pr_debug("ASYNC event= %d on device= %s\n", event->event,
		 srpt_sdev_name(sdev));

	switch (event->event) {
	case IB_EVENT_PORT_ERR:
		if (event->element.port_num <= sdev->device->phys_port_cnt) {
			sport = &sdev->port[event->element.port_num - 1];
			sport->lid = 0;
			sport->sm_lid = 0;
		}
		break;
	case IB_EVENT_PORT_ACTIVE:
	case IB_EVENT_LID_CHANGE:
	case IB_EVENT_PKEY_CHANGE:
	case IB_EVENT_SM_CHANGE:
	case IB_EVENT_CLIENT_REREGISTER:
	case IB_EVENT_GID_CHANGE:
		/* Refresh port data asynchronously. */
		if (event->element.port_num <= sdev->device->phys_port_cnt) {
			sport = &sdev->port[event->element.port_num - 1];
			if (!sport->lid && !sport->sm_lid)
				schedule_work(&sport->work);
		}
		break;
	default:
		pr_err("received unrecognized IB event %d\n",
		       event->event);
		break;
	}
}
