net: if: fix ND reachable calculation
net: if: fix ND reachable calculation

The reachable calculation for ND is using fractions combined with
integers and getting rounded to very small results (1ms or 0ms).
Let's split up the fraction into it's numerator and denominator
and perform the math in a better way to get the correct results.

Signed-off-by: Michael Scott <17b9e1c64588c7fa6419b4d29dc1f4426279ba01@opensourcefoundries.com>

mbolivar/zephyr,ldts/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,mbolivar/zephyr,galak/zephyr,galak/zephyr,punitvara/zephyr,Vudentz/zephyr,zephyriot/zephyr,galak/zephyr,GiulianoFranchetto/zephyr,mbolivar/zephyr,finikorg/zephyr,GiulianoFranchetto/zephyr,mbolivar/zephyr,aceofall/zephyr-iotos,nashif/zephyr,Vudentz/zephyr,ldts/zephyr,punitvara/zephyr,zephyriot/zephyr,explora26/zephyr,punitvara/zephyr,zephyriot/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,aceofall/zephyr-iotos,nashif/zephyr,explora26/zephyr,zephyriot/zephyr,finikorg/zephyr,kraj/zephyr,finikorg/zephyr,punitvara/zephyr,ldts/zephyr,nashif/zephyr,finikorg/zephyr,kraj/zephyr,zephyrproject-rtos/zephyr,explora26/zephyr,zephyrproject-rtos/zephyr,ldts/zephyr,explora26/zephyr,GiulianoFranchetto/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,kraj/zephyr,ldts/zephyr,Vudentz/zephyr,GiulianoFranchetto/zephyr,galak/zephyr,kraj/zephyr,aceofall/zephyr-iotos,kraj/zephyr,aceofall/zephyr-iotos,finikorg/zephyr,nashif/zephyr,aceofall/zephyr-iotos,nashif/zephyr,explora26/zephyr,Vudentz/zephyr,punitvara/zephyr,mbolivar/zephyr,zephyriot/zephyr