f2fs: fix to detect inconsistent nat entry during truncation

As Roman Smirnov reported as below:

"
There is a possible bug in f2fs_truncate_inode_blocks():

    if (err < 0 && err != -ENOENT)
    			goto fail;
        ...
        offset[1] = 0;
        offset[0]++;
        nofs += err;

If err = -ENOENT then nofs will sum with an error code,
which is strange behaviour. Also if nofs < ENOENT this will
cause an overflow. err will be equal to -ENOENT with the
following call stack:

truncate_nodes()
  f2fs_get_node_page()
     __get_node_page()
        read_node_page()
"

If nat is corrupted, truncate_nodes() may return -ENOENT, and
f2fs_truncate_inode_blocks() doesn't handle such error correctly,
fix it.

Reported-by: Roman Smirnov <[email protected]>
Closes: https://lore.kernel.org/linux-f2fs-devel/[email protected]
Signed-off-by: Chao Yu <[email protected]>
Signed-off-by: Jaegeuk Kim <[email protected]>

fs/f2fs/node.c

@@ -1187,7 +1187,17 @@ int f2fs_truncate_inode_blocks(struct inode *inode, pgoff_t from)
 		default:
 			BUG();
 		}
-		if (err < 0 && err != -ENOENT)
+		if (err == -ENOENT) {
+			set_sbi_flag(F2FS_P_SB(page), SBI_NEED_FSCK);
+			f2fs_handle_error(sbi, ERROR_INVALID_BLKADDR);
+			f2fs_err_ratelimited(sbi,
+				"truncate node fail, ino:%lu, nid:%u, "
+				"offset[0]:%d, offset[1]:%d, nofs:%d",
+				inode->i_ino, dn.nid, offset[0],
+				offset[1], nofs);
+			err = 0;
+		}
+		if (err < 0)
 			goto fail;
 		if (offset[1] == 0 &&
 				ri->i_nid[offset[0] - NODE_DIR1_BLOCK]) {