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>
---
 include/linux/scatterlist.h |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff -r b3aec596b841 include/linux/scatterlist.h
--- a/include/linux/scatterlist.h	Mon Jan 07 12:43:56 2008 +1100
+++ b/include/linux/scatterlist.h	Mon Jan 07 15:01:51 2008 +1100
@@ -188,8 +188,8 @@ static inline void sg_chain(struct scatt
 	/*
 	 * 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