[SCSI] st: convert do_load_unload to use st_scsi_kern_execute

This replaces st_do_scsi in do_load_unload (START STOP) with
st_scsi_kern_execute.

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>