md/raid1: store behind-write pages in bi_vecs.

When performing write-behind we allocate pages to store the data
during write.
Previously we just keep a list of pages.  Now we keep a list of
bi_vec which includes offset and size.
This means that the r1bio has complete information to create a new
bio which will be needed for retrying after write errors.

Signed-off-by: NeilBrown <neilb@suse.de>
Reviewed-by: Namhyung Kim <namhyung@gmail.com>
diff --git a/drivers/md/raid1.h b/drivers/md/raid1.h
index f81360d..b788547 100644
--- a/drivers/md/raid1.h
+++ b/drivers/md/raid1.h
@@ -101,7 +101,7 @@
 
 	struct list_head	retry_list;
 	/* Next two are only valid when R1BIO_BehindIO is set */
-	struct page		**behind_pages;
+	struct bio_vec		*behind_bvecs;
 	int			behind_page_count;
 	/*
 	 * if the IO is in WRITE direction, then multiple bios are used.