Allow checksum benchmarking during module load to be deferred until after boot, such as during system idle time or allow it to be disabled or delayed via a module parameter.

During boot on FreeBSD, the checksum benchmarking introduces a noticeable delay. On my system, it takes 1.06s, which is around 15% of total kernel initialization time (7.04s). Deferring this benchmarking would significantly reduce boot time, especially on systems where boot performance is critical.

Additional context
The issue seems related to this commit: 985c33b
This could be improved by either:

• Deferring the benchmark to a later point (e.g., post-boot idle)
• Adding a module parameter to control when or if the benchmark runs.