usb: gadget: f_fs: do not set cancel function on synchronous {read,write}

do not try to set cancel function in synchronous operations in
ffs_epfile_{read,write}_iter.

Cc: <stable@vger.kernel.org> # v4.0+
Acked-by: NAl Viro <viro@ZenIV.linux.org.uk>
Signed-off-by: NRui Miguel Silva <rui.silva@linaro.org>
Signed-off-by: NFelipe Balbi <balbi@ti.com>

drivers/usb/gadget/function/f_fs.c

@@ -924,6 +924,7 @@ static ssize_t ffs_epfile_write_iter(struct kiocb *kiocb, struct iov_iter *from)
 
 	kiocb->private = p;
 
+	if (p->aio)
 		kiocb_set_cancel_fn(kiocb, ffs_aio_cancel);
 
 	res = ffs_epfile_io(kiocb->ki_filp, p);
@@ -968,6 +969,7 @@ static ssize_t ffs_epfile_read_iter(struct kiocb *kiocb, struct iov_iter *to)
 
 	kiocb->private = p;
 
+	if (p->aio)
 		kiocb_set_cancel_fn(kiocb, ffs_aio_cancel);
 
 	res = ffs_epfile_io(kiocb->ki_filp, p);