init_main: Fix logging of the SYSINIT process

[zlei]

Summary

The current subsystem, aka sip->subsystem, should be logged, rather
than the last one.

Fixes: 5a8fceb3bd9f boottrace: trace annotations for startup and shutdown
MFC after: 1 week

Test Plan

Build kernel with option VERBOSE_SYSINIT, turn the knob debug.verbose_sysinit on and boot.

[mhorne]

Thanks!