[SCSI] st: integrate st_scsi_kern_execute and st_do_scsi

This integrates st_scsi_kern_execute and st_do_scsi. IOW, it removes
st_scsi_kern_execute. Then st has a single function, st_do_scsi, to
perform SCSI commands.

Signed-off-by: Kai Makisara <Kai.Makisara@kolumbus.fi>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>