f2fs: truncate data blocks for orphan inode

As Hu reported, F2FS has a space leak problem, when conducting:

1) format a 4GB f2fs partition
2) dd a 3G file,
3) unlink it.

So, when doing f2fs_drop_inode(), we need to truncate data blocks
before skipping it.
We can also drop unused caches assigned to each inode.
Reported-by: hujianyang <hujianyang@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/super.c

@@ -431,8 +431,30 @@ static int f2fs_drop_inode(struct inode *inode)
 	 *    - f2fs_gc -> iput -> evict
 	 *       - inode_wait_for_writeback(inode)
 	 */
-	if (!inode_unhashed(inode) && inode->i_state & I_SYNC)
+	if (!inode_unhashed(inode) && inode->i_state & I_SYNC) {
+		if (!inode->i_nlink && !is_bad_inode(inode)) {
+			spin_unlock(&inode->i_lock);
+
+			/* some remained atomic pages should discarded */
+			if (f2fs_is_atomic_file(inode))
+				commit_inmem_pages(inode, true);
+
+			sb_start_intwrite(inode->i_sb);
+			i_size_write(inode, 0);
+
+			if (F2FS_HAS_BLOCKS(inode))
+				f2fs_truncate(inode);
+
+			sb_end_intwrite(inode->i_sb);
+
+#ifdef CONFIG_F2FS_FS_ENCRYPTION
+			if (F2FS_I(inode)->i_crypt_info)
+				f2fs_free_encryption_info(inode);
+#endif
+			spin_lock(&inode->i_lock);
+		}
 		return 0;
+	}
 	return generic_drop_inode(inode);
 }