rtld: Fix a couple of bugs around the unloading of ELF filters.

• Pass the correct object to unload_filtees().
• Use a marker to restart iteration after unload_filtees() has returned. It calls dlclose() and may recursively remove entries from the global object list, so TAILQ_FOREACH_SAFE is not sufficient.

Reviewed by: kib
MFC after: 2 weeks
Sponsored by: Dell EMC Isilon