[SCSI] st: add st_scsi_execute helper function

st_scsi_execute is a helper function to perform SCSI commands
involving data transfer between user and kernel space (st_read and
st_write).

It's the future plan to combine this with st_scsi_kern_execute helper
function.

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>
diff --git a/drivers/scsi/st.h b/drivers/scsi/st.h
index 74748ab..77302fa 100644
--- a/drivers/scsi/st.h
+++ b/drivers/scsi/st.h
@@ -29,6 +29,7 @@
 	int result;
 	struct scsi_tape *stp;
 	struct completion *waiting;
+	struct bio *bio;
 };
 
 /* The tape buffer descriptor. */