staging: comedi: fix memory leak for saved channel list

commit c8cad4c8 upstream.

When `do_cmd_ioctl()` allocates memory for the kernel copy of a channel
list, it frees any previously allocated channel list in
`async->cmd.chanlist` and replaces it with the new one.  However, if the
device is ever removed (or "detached") the cleanup code in
`cleanup_device()` in "drivers.c" does not free this memory so it is
lost.

A sensible place to free the kernel copy of the channel list is in
`do_become_nonbusy()` as at that point the comedi asynchronous command
associated with the channel list is no longer valid.  Free the channel
list in `do_become_nonbusy()` instead of `do_cmd_ioctl()` and clear the
pointer to prevent it being freed more than once.

Note that `cleanup_device()` could be called at an inappropriate time
while the comedi device is open, but that's a separate bug not related
to this this patch.
Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/comedi/comedi_fops.c

@@ -1088,7 +1088,6 @@ static int do_cmd_ioctl(struct comedi_device *dev,
 		goto cleanup;
 	}
 
-	kfree(async->cmd.chanlist);
 	async->cmd = user_cmd;
 	async->cmd.data = NULL;
 	/* load channel/gain list */
@@ -1833,6 +1832,8 @@ void do_become_nonbusy(struct comedi_device *dev, struct comedi_subdevice *s)
 	if (async) {
 		comedi_reset_async_buf(async);
 		async->inttrig = NULL;
+		kfree(async->cmd.chanlist);
+		async->cmd.chanlist = NULL;
 	} else {
 		printk(KERN_ERR
 		       "BUG: (?) do_become_nonbusy called with async=0\n");