sched_4bsd: ESTCPULIM(): Allow any value in the timeshare range

The current formula wastes queues and degrades usage estimation
precision, since any increase of ticks that goes over 40 priorities (so,
8 * 40) is clamped to the last of these 40 levels (the nice value is
subsequently added to that number to get the final priority level).

Allow 'ts_estcpu' to grow up to a value corresponding to the greatest
(i.e., lowest) priority of the timeshare range.

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