f2fs: drop obsolete node page when it is truncated

If a node page is trucated, we'd better drop the page in the node_inode's page
cache for better memory footprint.
Signed-off-by: Jaegeuk Kim <jaegeuk.kim@samsung.com>

fs/f2fs/node.c

@@ -518,6 +518,10 @@ static void truncate_node(struct dnode_of_data *dn)
 	F2FS_SET_SB_DIRT(sbi);
 
 	f2fs_put_page(dn->node_page, 1);
+
+	invalidate_mapping_pages(NODE_MAPPING(sbi),
+			dn->node_page->index, dn->node_page->index);
+
 	dn->node_page = NULL;
 	trace_f2fs_truncate_node(dn->inode, dn->nid, ni.blk_addr);
 }