[SCSI] st: convert non-dio path to use st_scsi_execute

This patch converts the non-dio path (fragment buffer path) to use
st_scsi_execute. IOW, it removes scsi_execute_async in the non-dio
path.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Acked-by: Kai Makisara <Kai.Makisara@kolumbus.fi>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>