comedi: Fix memory leak in compat_insnlist()

`compat_insnlist()` handles the 32-bit version of the `COMEDI_INSNLIST`
ioctl (whenwhen `CONFIG_COMPAT` is enabled).  It allocates memory to
temporarily hold an array of `struct comedi_insn` converted from the
32-bit version in user space.  This memory is only being freed if there
is a fault while filling the array, otherwise it is leaked.

Add a call to `kfree()` to fix the leak.

Fixes: b8d47d88 ("comedi: get rid of compat_alloc_user_space() mess in COMEDI_INSNLIST compat")
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: linux-staging@lists.linux.dev
Cc: <stable@vger.kernel.org> # 5.13+
Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Link: https://lore.kernel.org/r/20210916145023.157479-1-abbotti@mev.co.ukSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/comedi/comedi_fops.c

@@ -3090,6 +3090,7 @@ static int compat_insnlist(struct file *file, unsigned long arg)
 	mutex_lock(&dev->mutex);
 	rc = do_insnlist_ioctl(dev, insns, insnlist32.n_insns, file);
 	mutex_unlock(&dev->mutex);
+	kfree(insns);
 	return rc;
 }