f2fs: return error when accessing insane flie offset

If file offset is insane, we have to return error instead of kernel panic.
Reported-by: NEric Zhang <followme999@163.com>
Signed-off-by: NJaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/node.c

@@ -555,7 +555,7 @@ static int get_node_path(struct inode *inode, long block,
 		level = 3;
 		goto got;
 	} else {
-		BUG();
+		return -E2BIG;
 	}
 got:
 	return level;
@@ -579,6 +579,8 @@ int get_dnode_of_data(struct dnode_of_data *dn, pgoff_t index, int mode)
 	int err = 0;
 
 	level = get_node_path(dn->inode, index, offset, noffset);
+	if (level < 0)
+		return level;
 
 	nids[0] = dn->inode->i_ino;
 	npage[0] = dn->inode_page;
@@ -878,6 +880,8 @@ int truncate_inode_blocks(struct inode *inode, pgoff_t from)
 	trace_f2fs_truncate_inode_blocks_enter(inode, from);
 
 	level = get_node_path(inode, from, offset, noffset);
+	if (level < 0)
+		return level;
 
 	page = get_node_page(sbi, inode->i_ino);
 	if (IS_ERR(page)) {