Input: don't modify the id of ioctl-provided ff effect on upload failure

If a new (id == -1) ff effect was uploaded from userspace,
ff-core.c::input_ff_upload() will have assigned a positive number to the
new effect id.  Currently, evdev.c::evdev_do_ioctl() will save this new id
to userspace, regardless of whether the upload succeeded or not.

On upload failure, this can be confusing because the dev->ff->effects[]
array will not contain an element at the index of that new effect id.

This patch fixes this by leaving the id unchanged after upload fails.

Note: Unfortunately applications should still expect changed effect id for
quite some time.

This has been discussed on:
http://www.mail-archive.com/linux-input@vger.kernel.org/msg08513.html
("ff-core effect id handling in case of a failed effect upload")

Suggested-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: Elias Vanderstuyft <elias.vds@gmail.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>

drivers/input/evdev.c

@@ -954,11 +954,13 @@ static long evdev_do_ioctl(struct file *file, unsigned int cmd,
 			return -EFAULT;
 
 		error = input_ff_upload(dev, &effect, file);
+		if (error)
+			return error;
 
 		if (put_user(effect.id, &(((struct ff_effect __user *)p)->id)))
 			return -EFAULT;
 
-		return error;
+		return 0;
 	}
 
 	/* Multi-number variable-length handlers */