sched_4bsd: Move ESTCPULIM() after its macro dependencies

No functional change (intended).

Also makes the comment about INVERSE_ESTCPU_WEIGHT() adjacent to its
definition.

MFC after: 1 month
Event: Kitchener-Waterloo Hackathon 202506
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D45392