usb: gadget: ffs: Execute copy_to_user() with USER_DS set

When using a AIO read() operation on the function FS gadget driver a URB is
submitted asynchronously and on URB completion the received data is copied
to the userspace buffer associated with the read operation.

This is done from a kernel worker thread invoking copy_to_user() (through
copy_to_iter()). And while the user space process memory is made available
to the kernel thread using use_mm(), some architecture require in addition
to this that the operation runs with USER_DS set. Otherwise the userspace
memory access will fail.

For example on ARM64 with Privileged Access Never (PAN) and User Access
Override (UAO) enabled the following crash occurs.

	Internal error: Accessing user space memory with fs=KERNEL_DS: 9600004f [#1] SMP
	Modules linked in:
	CPU: 2 PID: 1636 Comm: kworker/2:1 Not tainted 4.9.0-04081-g8ab2dfb-dirty #487
	Hardware name: ZynqMP ZCU102 Rev1.0 (DT)
	Workqueue: events ffs_user_copy_worker
	task: ffffffc87afc8080 task.stack: ffffffc87a00c000
	PC is at __arch_copy_to_user+0x190/0x220
	LR is at copy_to_iter+0x78/0x3c8
	[...]
	[<ffffff800847b790>] __arch_copy_to_user+0x190/0x220
	[<ffffff80086f25d8>] ffs_user_copy_worker+0x70/0x130
	[<ffffff80080b8c64>] process_one_work+0x1dc/0x460
	[<ffffff80080b8f38>] worker_thread+0x50/0x4b0
	[<ffffff80080bf5a0>] kthread+0xd8/0xf0
	[<ffffff8008083680>] ret_from_fork+0x10/0x50

Address this by placing a set_fs(USER_DS) before of the copy operation
and revert it again once the copy operation has finished.

This patch is analogous to commit d7ffde35 ("vhost: use USER_DS in
vhost_worker thread") which addresses the same underlying issue.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>