block: Fix elevator name declaration

The elevator_name field in struct elevator_type is declared as an array
of characters (ELV_NAME_MAX size) but in practice used as a string
pointer with its initialization done statically within each
elevator elevator_type structure declaration.

Change the declaration of elevator_name to the more appropriate
"const char *" type.

Acked-by: Marcos Paulo de Souza <marcos.souza.org@gmail.com>
Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>