staging: lustre: fix sparse warning about different address spaces

fixed the following sparse warning by adding proper cast:
drivers/staging//lustre/lustre/obdclass/obd_config.c:1055:74: warning: incorrect type in argument 2 (different address spaces)
drivers/staging//lustre/lustre/obdclass/obd_config.c:1055:74:    expected char const [noderef] <asn:1>*<noident>
drivers/staging//lustre/lustre/obdclass/obd_config.c:1055:74:    got char *[assigned] sval
Signed-off-by: Mario Bambagini <mario.bambagini@gmail.com>
Acked-by: Oleg Drokin <oleg.drokin@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/lustre/lustre/obdclass/obd_config.c

@@ -1052,7 +1052,8 @@ int class_process_proc_param(char *prefix, struct lprocfs_vars *lvars,
 
 					oldfs = get_fs();
 					set_fs(KERNEL_DS);
-					rc = var->fops->write(&fakefile, sval,
+					rc = var->fops->write(&fakefile,
+						(const char __user *)sval,
 								vallen, NULL);
 					set_fs(oldfs);
 				}