Btrfs: remove repeated eb->pages check in, disk-io.c/csum_dirty_buffer

In csum_dirty_buffer, we first get eb from page->private.
Then we check if the page is the first page of eb. Later
we check it again. Remove the repeated check here.
Signed-off-by: NWang Sheng-Hui <shhuiw@gmail.com>

fs/btrfs/disk-io.c

@@ -433,10 +433,6 @@ static int csum_dirty_buffer(struct btrfs_root *root, struct page *page)
 		WARN_ON(1);
 		return 0;
 	}
-	if (eb->pages[0] != page) {
-		WARN_ON(1);
-		return 0;
-	}
 	if (!PageUptodate(page)) {
 		WARN_ON(1);
 		return 0;