staging: comedi: fix memory leak / bad pointer freeing for chanlist

As a follow-up to commit 6cab7a37 ("staging: comedi: (regression)
channel list must be set for COMEDI_CMD ioctl"), Hartley Sweeten pointed
out another couple of bugs stemming from commit 6cab7a37 ("staging:
comedi: comedi_fops: introduce __comedi_get_user_chanlist()").

Firstly, `do_cmdtest_ioctl()` never frees the kernel copy of the user
chanlist allocated by `__comedi_get_user_chanlist()`, so that memory is
leaked.  Fix it by freeing the allocated kernel memory pointed to by
`cmd.chanlist` before that pointer is overwritten with its original
pointer to user memory before `cmd` is copied back to user-space.

Secondly, if `__comedi_get_user_chanlist()` returns an error,
`cmd->chanlist` is left unchanged and in fact will be a pointer to user
memory.  This causes `do_cmd_ioctl()` to `goto cleanup` and call
`do_become_nonbusy()` which would attempt to free the memory pointed to
by the user-space pointer.  Fix it by setting `cmd->chanlist` to NULL at
the start of `__comedi_get_user_chanlist()`.

Fixes: c6cd0eef ("staging: comedi: comedi_fops: introduce __comedi_get_user_chanlist()")
Reported-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Cc: <stable@vger.kernel.org> # 3.15.y 3.16.y 3.17.y: 6cab7a37
Cc: <stable@vger.kernel.org> # 3.15.y 3.16.y 3.17.y
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/comedi/comedi_fops.c

@@ -1462,6 +1462,7 @@ static int __comedi_get_user_chanlist(struct comedi_device *dev,
 	unsigned int *chanlist;
 	int ret;
 
+	cmd->chanlist = NULL;
 	chanlist = memdup_user(user_chanlist,
 			       cmd->chanlist_len * sizeof(unsigned int));
 	if (IS_ERR(chanlist))
@@ -1615,6 +1616,8 @@ static int do_cmdtest_ioctl(struct comedi_device *dev,
 
 	ret = s->do_cmdtest(dev, s, &cmd);
 
+	kfree(cmd.chanlist);	/* free kernel copy of user chanlist */
+
 	/* restore chanlist pointer before copying back */
 	cmd.chanlist = (unsigned int __force *)user_chanlist;