Loading glibc in a new namespace fails vtable check

[Florian Weimer]

* Moshe Rubin:

>   // Get the address of fileno() from the new glibc
>   int (*libc_fileno)(FILE *fp);
>   libc_fileno = reinterpret_cast<decltype(libc_fileno)>(dlsym(handle, "fileno"));
>
>   int fd = libc_fileno(stderr);

I think fileno does not go through vtable dispatch, so it doesn't
perform vtable verfication.

Thanks,
Florian