staging/lustre: Properly cast ll_getname argument to __user in ll_dir_ioctl

This makes sparse happy.
Signed-off-by: Oleg Drokin <green@linuxhacker.ru>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/lustre/lustre/llite/dir.c

@@ -1448,7 +1448,7 @@ static long ll_dir_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
 
 		if (cmd == IOC_MDC_GETFILEINFO ||
 		    cmd == IOC_MDC_GETFILESTRIPE) {
-			filename = ll_getname((const char *)arg);
+			filename = ll_getname((const char __user *)arg);
 			if (IS_ERR(filename))
 				return PTR_ERR(filename);