f128150d43
Preserve errno because this function is often used like
the following. The caller wants to report the error from the main
operation (`lseek` in this example), not from fd_object_release.
```
off_t ret = lseek(fd_number(fo), offset, nwhence);
fd_object_release(fo);
if (ret < 0)
return convert_errno(errno);
```