iio: buffer: return 0 for buffer getfd ioctl handler

As Lars pointed out, we could either return the FD vs memcpy-ing it to the
userspace data object.

However, this comment exposed a bug. We should return 0 or negative from
these ioctl() handlers. Because an ioctl() handler can also return
IIO_IOCTL_UNHANDLED (which is positive 1), which means that the ioctl()
handler doesn't support this ioctl number. Positive 1 could also be a valid
FD number in some corner cases.

The reason we did this is to be able to differentiate between an error
code and an unsupported ioctl number; for unsupported ioctl numbers, the
main loop should keep going.

Maybe we should change this to a higher negative number, to avoid such
cases when/if we add more ioctl() handlers.

Cc: Lars-Peter Clausen <lars@metafoo.de>
Fixes: f73f7f4da5 ("iio: buffer: add ioctl() to support opening extra buffers for IIO device")
Signed-off-by: Alexandru Ardelean <aardelean@deviqon.com>
Link: https://lore.kernel.org/r/20210322084135.17536-1-aardelean@deviqon.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/industrialio-buffer.c

@@ -1442,7 +1442,7 @@ static long iio_device_buffer_getfd(struct iio_dev *indio_dev, unsigned long arg
 		goto error_free_ib;
 	}
 
-	return fd;
+	return 0;
 
 error_free_ib:
 	kfree(ib);