Btrfs: don't null pointer deref on abort

I'm sorry, theres no excuse for this sort of work.  We need to use
root->leafsize since eb may be NULL.  Thanks,

Signed-off-by: Josef Bacik <jbacik@fusionio.com>
diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index 72b1727..e8b29da 100644
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -3808,7 +3808,7 @@
 		while (start <= end) {
 			eb = btrfs_find_tree_block(root, start,
 						   root->leafsize);
-			start += eb->len;
+			start += root->leafsize;
 			if (!eb)
 				continue;
 			wait_on_extent_buffer_writeback(eb);