bsg: Kconfig updates

This updates bsg entry in Kconfig:

- bsg supports sg v4
- bsg depends on SCSI
- it might be better to mark it experimental for a while

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>