sys/systm.h: use __nodiscard consistently and correctly

nodiscard is closer to (and sometimes expands to) nodiscard from
C23 and C++17 so prefer it to the homegrown result_use_check and put
it in the right place so it is correct when expanded to nodiscard.

Reviewed by: markj, emaste
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D57882