CFT: if_bridge performance improvements

Hi,

Thanks to support from The FreeBSD Foundation I’ve been able to work 
on improving the throughput of if_bridge.
It changes the (data path) locking to use the NET_EPOCH infrastructure. 
Benchmarking shows substantial improvements (x5 in test setups).

This work is ready for wider testing now.

It’s under review here: https://reviews.freebsd.org/D24250

Patch for CURRENT: https://reviews.freebsd.org/D24250?download=true
Patches for stable/12: 
https://people.freebsd.org/~kp/if_bridge/stable_12/

I’m not currently aware of any panics or issues resulting from these 
patches.

Do note that if you run a Bhyve + tap on bridges setup the tap code 
suffers from a similar bottleneck and you will likely not see major 
improvements in single VM to host throughput. I would expect, but have 
not tested, improvements in overall throughput (i.e. when multiple VMs 
send traffic at the same time).

Best regards,
Kristof