staging: comedi: avoid bad truncation of a size_t in comedi_read()

At one point in `comedi_read()`, the variable `n` gets assigned to the
minimum of the parameter `nbytes` and the amount of readable buffer
space `m`.  The way that is done currently is unsafe in the unlikely
case that `nbytes` exceeds `UINT_MAX`, so fix it.
Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Reviewed-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/comedi/comedi_fops.c

@@ -2493,13 +2493,10 @@ static ssize_t comedi_read(struct file *file, char __user *buf, size_t nbytes,
 	while (nbytes > 0 && !retval) {
 		set_current_state(TASK_INTERRUPTIBLE);
 
-		n = nbytes;
-
 		m = comedi_buf_read_n_available(s);
 		if (async->buf_read_ptr + m > async->prealloc_bufsz)
 			m = async->prealloc_bufsz - async->buf_read_ptr;
-		if (m < n)
-			n = m;
+		n = min_t(size_t, m, nbytes);
 
 		if (n == 0) {
 			unsigned runflags = comedi_get_subdevice_runflags(s);