linuxkpi: Add hrtimer_setup()

hrtimer_setup() replaces hrtimer_init() in Linux 6.15. The API and
the role are the same, except:

• it takes the function callback as argument
• if function is NULL, it assigns a default callback

At the same time, we hide hrtimer_init() if LINUXKPI_VERSION is
greater than or equal to 61500 because it was dropped in Linux 6.15.

Reviewed by: bz
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D58757