config_intrhook: Move from TAILQ to STAILQ and padding

config_intrhook doesn't need to be a two-pointer TAILQ. We rarely add/delete
from this and so those need not be optimized. Instaed, use the one-pointer
STAILQ plus a uintptr_t to be used as a flags word. This will allow these
changes to be MFC'd to 12 and 13 to fix a race in removable devices.

Feedback from: jhb
Reviewed by: mav
Differential Revision: https://reviews.freebsd.org/D29004