[SCSI] osst: replace scsi_execute_async with the block layer API

This replaces scsi_execute_async with the block layer API. st does the
same thing so it might make sense to have something like libst (there
are other things that os and osst can share).

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Cc: Willem Riede <osst@riede.org>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>