init_main: Fix logging the subsystem of pre-loaded kernel modules

The pre-load, aka linker_preload() runs at the order of SI_SUB_KLD, but
a pre-loaded module may have SYSINITs that have startup order prior to
SI_SUB_KLD, e.g. TUNABLE_INT() / TUNABLE_LONG(), hence it is possible
that we run into abnormal orders.

Without this change, the subsystem of the pre-loaded kernel modules
will be melted into previous one. That is mostly harmless but confusing.

MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D47904

(cherry picked from commit 11f1dd193af57dfa5128eadc62b084502292a784)
(cherry picked from commit b2293f7f60c238e7a83551472aed9452cd017769)