Don't blatt first element of prv in sg_chain()

I realize that sg chaining is a ploy to make the rest of the kernel
devs feel the pain of the SCSI subsystem.  But this was a little
unsubtle.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Acked-by: Tejun Heo <htejun@gmail.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

include/linux/scatterlist.h

@@ -191,8 +191,8 @@ static inline void sg_chain(struct scatterlist *prv, unsigned int prv_nents,
 	/*
 	 * offset and length are unused for chain entry.  Clear them.
 	 */
-	prv->offset = 0;
-	prv->length = 0;
+	prv[prv_nents - 1].offset = 0;
+	prv[prv_nents - 1].length = 0;
 
 	/*
 	 * Set lowest bit to indicate a link pointer, and make sure to clear