Fix conditional/message for max lba for sg ioengine

Fix conditional/message for max lba for sg ioengine

Whether sg ioengine uses 10 or 16 bytes read/write command (by 5ad7be56)
is determined by ->prep() depending on lba to read/write vs 0xFFFFFFFFULL,
so let the message show it depends.

Also remove unused sgio_data::max_lba which is essentially the same
(divide by bs minus 1) as fio_file::real_file_size.

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