Staging: lustre: Avoid using 0 instead of NULL

Fixed the following sparse complaints:

drivers/staging/lustre/lustre/ptlrpc/../../lustre/ldlm/ldlm_pool.c:755:9:
	warning: Using plain integer as NULL pointer
drivers/staging/lustre/lustre/ptlrpc/../../lustre/ldlm/ldlm_pool.c:757:9:
	warning: Using plain integer as NULL pointer
drivers/staging/lustre/lustre/ptlrpc/../../lustre/ldlm/ldlm_pool.c:758:9:
	warning: Using plain integer as NULL pointer
drivers/staging/lustre/lustre/ptlrpc/../../lustre/ldlm/ldlm_pool.c:759:9:
	warning: Using plain integer as NULL pointer
drivers/staging/lustre/lustre/ptlrpc/../../lustre/ldlm/ldlm_pool.c:760:9:
	warning: Using plain integer as NULL pointer
drivers/staging/lustre/lustre/ptlrpc/../../lustre/ldlm/ldlm_pool.c:762:9:
	warning: Using plain integer as NULL pointer
drivers/staging/lustre/lustre/ptlrpc/../../lustre/ldlm/ldlm_pool.c:764:9:
	warning: Using plain integer as NULL pointer
drivers/staging/lustre/lustre/ptlrpc/../../lustre/ldlm/ldlm_pool.c:765:9:
	warning: Using plain integer as NULL pointer
drivers/staging/lustre/lustre/ptlrpc/../../lustre/ldlm/ldlm_pool.c:766:9:
	warning: Using plain integer as NULL pointer
drivers/staging/lustre/lustre/ptlrpc/../../lustre/ldlm/ldlm_pool.c:768:9:
	warning: Using plain integer as NULL pointer
Signed-off-by: Fabio Falzoi <fabio.falzoi84@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/lustre/lustre/ldlm/ldlm_pool.c

@@ -718,7 +718,7 @@ LPROC_SEQ_FOPS_RO(lprocfs_grant_speed);
 		snprintf(var_name, MAX_STRING_SIZE, #name);	\
 		pool_vars[0].data = var;			\
 		pool_vars[0].fops = ops;			\
-		lprocfs_add_vars(pl->pl_proc_dir, pool_vars, 0);\
+		lprocfs_add_vars(pl->pl_proc_dir, pool_vars, NULL);\
 	} while (0)
 
 static int ldlm_pool_proc_init(struct ldlm_pool *pl)