Staging: comedi: Return -EFAULT if copy_to_user() fails

Return -EFAULT on error instead of the number of bytes remaining to be
copied.

Fixes: bac42fb21259 ("comedi: get rid of compat_alloc_user_space() mess in COMEDI_CMD{,TEST} compat")
Signed-off-by: Dan Carpenter <[e-mail]>
Cc: stable <[e-mail]>
Link: https://lore.kernel.org/r/X8c3pfwFy2jpy4BP@mwanda
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/staging/comedi/comedi_fops.c

···
 	v32.chanlist_len = cmd->chanlist_len;
 	v32.data = ptr_to_compat(cmd->data);
 	v32.data_len = cmd->data_len;
-	return copy_to_user(cmd32, &v32, sizeof(v32));
+	if (copy_to_user(cmd32, &v32, sizeof(v32)))
+		return -EFAULT;
+	return 0;
 }
 
 /* Handle 32-bit COMEDI_CMD ioctl. */