md: raid10: avoid direct access to bvec table in handle_reshape_read_error

All reshape I/O share pages from 1st copy device, so just use that pages
for avoiding direct access to bvec table in handle_reshape_read_error.
Signed-off-by: NMing Lei <tom.leiming@gmail.com>
Signed-off-by: NShaohua Li <shli@fb.com>

drivers/md/raid10.c

@@ -4653,7 +4653,10 @@ static int handle_reshape_read_error(struct mddev *mddev,
 	struct r10bio *r10b = &on_stack.r10_bio;
 	int slot = 0;
 	int idx = 0;
-	struct bio_vec *bvec = r10_bio->master_bio->bi_io_vec;
+	struct page **pages;
+
+	/* reshape IOs share pages from .devs[0].bio */
+	pages = get_resync_pages(r10_bio->devs[0].bio)->pages;
 
 	r10b->sector = r10_bio->sector;
 	__raid10_find_phys(&conf->prev, r10b);
@@ -4682,7 +4685,7 @@ static int handle_reshape_read_error(struct mddev *mddev,
 			success = sync_page_io(rdev,
 					       addr,
 					       s << 9,
-					       bvec[idx].bv_page,
+					       pages[idx],
 					       REQ_OP_READ, 0, false);
 			rdev_dec_pending(rdev, mddev);
 			rcu_read_lock();