f2fs: skip get_node_page if locked node page is passed

If get_dnode_of_data gets a locked node page, let's skip redundant
get_node_page calls.
This is for the futher enhancement.
Signed-off-by: Jaegeuk Kim <jaegeuk.kim@samsung.com>

fs/f2fs/node.c

@@ -408,10 +408,13 @@ int get_dnode_of_data(struct dnode_of_data *dn, pgoff_t index, int mode)
 	level = get_node_path(index, offset, noffset);
 
 	nids[0] = dn->inode->i_ino;
+	npage[0] = dn->inode_page;
+
+	if (!npage[0]) {
 		npage[0] = get_node_page(sbi, nids[0]);
 		if (IS_ERR(npage[0]))
 			return PTR_ERR(npage[0]);
-
+	}
 	parent = npage[0];
 	if (level != 0)
 		nids[1] = get_nid(parent, offset[0], true);