usb: gadget: ffs: handle I/O completion in-order

By submitting completed transfers to the system workqueue there is no
guarantee that completion events will be queued up in the correct order,
as in multi-processor systems there is a thread running for each
processor and the work items are not bound to a particular core.

This means that several completions are in the queue at the same time,
they may be processed in parallel and complete out of order, resulting
in data appearing corrupt when read by userspace.

Create a single-threaded workqueue for FunctionFS so that data completed
requests is passed to userspace in the order in which they complete.

Acked-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: John Keeping <john@metanate.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>

drivers/usb/gadget/function/u_fs.h

@@ -279,6 +279,7 @@ struct ffs_data {
 	}				file_perms;
 
 	struct eventfd_ctx *ffs_eventfd;
+	struct workqueue_struct *io_completion_wq;
 	bool no_disconnect;
 	struct work_struct reset_work;