lib/vfscore: Remove readdir_r result from TLS

lib/vfscore: Remove readdir_r result from TLS

The `result` structure of the `readdir_r` function will not need to be
part of the TLS, the `entry` structure alone is enough.

Signed-off-by: Stefan Jumarea <stefanjumarea02@gmail.com>
Reviewed-by: Radu Nichita <radunichita99@gmail.com>
Reviewed-by: Sergiu Moga <sergiu@unikraft.io>
Reviewed-by: Simon Kuenzer <simon@unikraft.io>
Approved-by: Simon Kuenzer <simon@unikraft.io>
Tested-by: Unikraft CI <monkey@unikraft.io>
GitHub-Closes: #963