LinuxKPI: implement mul_u64_u64_div_u64()

Implement mul_u64_u64_div_u64() for an updated iwlwifi driver (though
we do not yet use it there; it is used for in-kernel ptp on wifi).

Sponsored by: The FreeBSD Foundation

(cherry picked from commit b80ea452375f52a3ab7d82a9aef10da0d89985d9)

Unbalanced parentheses broke the build; re-balance.

Fixes: b80ea452375f ("LinuxKPI: implement mul_u64_u64_div_u64()")
(cherry picked from commit dabbbebcb0f5ced163454cd08b78e551b4a365b5)

linuxkpi math: fix kassert in math64.h

Include <sys/systm.h> in math64.h, so that KASSERT and bool are
defined, to allow compilation to succeed after
b80ea452375f52a3ab7d82a9aef10da0d89985d9 and dabbbebcb0f5...

(cherry picked from commit b6a61d6836d90ff2756d804eb981a02b0828f496)