depmod: cache info_list in struct mod

depmod: cache info_list in struct mod

The overall goal is to coalesce the accesses to a file that is the
backend of a module. This commit addresses the calls to
kmod_module_get_info(). Calling it earlier, while we are iterating the
modules allows us to free the struct kmod of each module much sooner. We
are still not freeing it since there are other places that must be
refactored first.

A nice side effect is that this commit reduces in ~33% the calls to
malloc(), giving a speedup of ~6% for cold caches (reproduced on only 1
laptop).