polling in 5.1-RC1 causes 100% CPU usage

[Damian Gerow]

I just upgraded a home firewall from 4.8-STABLE to 5.1-RC1.  In the process
of updating, I also brought in a new motherboard, CPU, and memory.  However,
all the NICs in the box are the same.

The problem is that with polling turned on, CPU usage sits at 100%.  As soon
as I turn /off/ polling, it drops.  This is very easily reproducible:

    # sysctl -w kern.polling.enable=1

Then do a 'systat -vm', and watch as the interrupt load jumps from about
0.5% to 18-19%.