elevator: use ELV_NAME_MAX instead of magic number 16 for chosen_elevator

We have ELV_NAME_MAX defined to 16, and hence we should use it
instead of the magic nubmer 16 for elevator's name string.
Signed-off-by: NWang Sheng-Hui <shhuiw@gmail.com>
Signed-off-by: NJens Axboe <jaxboe@fusionio.com>

block/elevator.c

@@ -182,7 +182,7 @@ static void elevator_attach(struct request_queue *q, struct elevator_queue *eq,
 	eq->elevator_data = data;
 }
 
-static char chosen_elevator[16];
+static char chosen_elevator[ELV_NAME_MAX];
 
 static int __init elevator_setup(char *str)
 {