kern_reboot: unconditionally call shutdown_reset()

Currently shutdown_reset() is registered as the final entry of the
shutdown_final event handler. However, if a panic occurs early in boot
before the event is registered (SI_SUB_INTRINSIC), we may end up
spinning in the subsequent infinite for loop and failing to reset
altogether. Instead we can simply call this function unconditionally.

Reviewed by: markj
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D37981