linuxkpi: Introduce module_param() of type hexint

In Linux, this affects how the value is formatted. In FreeBSD, this
remains an unsigned integer.

Reviewed by: manu
Approved by: manu
Differential Revision: https://reviews.freebsd.org/D36958

(cherry picked from commit 0b8a423d073309ab5f24501c2f6b3f6b2cd8f422)