nilfs2: simplify nilfs_get_page function

Implementation of nilfs_get_page() is a bit old as below:

 - A common read_mapping_page inline function is now available instead
   of its read_cache_page use.
 - wait_on_page_locked() use in the function is eliminable since
   read_cache_page function does the same thing through wait_on_page_read().
 - PageUptodate() check is eliminable for the same reason.

This renews nilfs_get_page() based on these points.
Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>

fs/nilfs2/dir.c

@@ -199,13 +199,10 @@ static void nilfs_check_page(struct page *page)
 static struct page *nilfs_get_page(struct inode *dir, unsigned long n)
 {
 	struct address_space *mapping = dir->i_mapping;
-	struct page *page = read_cache_page(mapping, n,
-				(filler_t *)mapping->a_ops->readpage, NULL);
+	struct page *page = read_mapping_page(mapping, n, NULL);
+
 	if (!IS_ERR(page)) {
-		wait_on_page_locked(page);
 		kmap(page);
-		if (!PageUptodate(page))
-			goto fail;
 		if (!PageChecked(page))
 			nilfs_check_page(page);
 		if (PageError(page))