[SCSI] st: fix kernel BUG at include/linux/scatterlist.h:59!

This is caused by a missing scatterlist initialisation (it only shows
up when sg list handling debugging is turned on).
Signed-off-by: NFUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Cc: Kai Makisara <Kai.Makisara@kolumbus.fi>
Signed-off-by: NJames Bottomley <James.Bottomley@HansenPartnership.com>

drivers/scsi/st.c

@@ -3611,6 +3611,7 @@ static struct st_buffer *
 
 	tb->dma = need_dma;
 	tb->buffer_size = got;
+	sg_init_table(tb->sg, max_sg);
 
 	return tb;
 }