kern_linker: Handle module-loading failures in preloaded .ko files

The runtime kernel loader, linker_load_file, unloads kernel files that
failed to load all of their modules. For consistency, treat preloaded
(loader.conf loaded) kernel files in the same way.

Reviewed by: kib
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D8200