link_elf_obj: Invoke fini callbacks

This is required for KASAN: when a module is unloaded, poisoned regions
(e.g., pad areas between global variables) are left as such, so if they
are reused as KLDs are loaded, false positives can arise.

Reported by: pho, Jenkins
Reviewed by: kib
Sponsored by: The FreeBSD Foundation

(cherry picked from commit 9e575fadf491152fb3445d4837d49a9cb87dd6e2)