RDMA/ib_srp: Fix a deadlock

Remove the flush_workqueue(system_long_wq) call since flushing
system_long_wq is deadlock-prone and since that call is redundant with a
preceding cancel_work_sync()

Link: https://lore.kernel.org/r/20220215210511.28303-3-bvanassche@acm.org
Fixes: ef6c49d8 ("IB/srp: Eliminate state SRP_TARGET_DEAD")
Reported-by: syzbot+831661966588c802aae9@syzkaller.appspotmail.com
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Reviewed-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>

drivers/infiniband/ulp/srp/ib_srp.c

@@ -4047,9 +4047,11 @@ static void srp_remove_one(struct ib_device *device, void *client_data)
 		spin_unlock(&host->target_lock);
 
 		/*
-		 * Wait for tl_err and target port removal tasks.
+		 * srp_queue_remove_work() queues a call to
+		 * srp_remove_target(). The latter function cancels
+		 * target->tl_err_work so waiting for the remove works to
+		 * finish is sufficient.
 		 */
-		flush_workqueue(system_long_wq);
 		flush_workqueue(srp_remove_wq);
 
 		kfree(host);