Rename FIO_TYPE_BD to FIO_TYPE_BLOCK

Rename FIO_TYPE_BD to FIO_TYPE_BLOCK

Since character device is FIO_TYPE_CHAR, BLOCK (or BLK) makes
better sense for block device.

This may break external ioengines on compile-time, but ioengines
usually don't refer to file type which are regfile/blk/chr/pipe
(which I believe makes sense to commit this change as there's
been changes that break not only api but also abi).

Signed-off-by: Tomohiro Kusumi <tkusumi@tuxera.com>
Signed-off-by: Jens Axboe <axboe@fb.com>