kernel linkers: Eliminate unneeded vnet propagation

[zlei]

Summary

1. The module preload happens before vnet0 creating, thus vnet_data_copy() will say empty vnet list.
2. For modules loaded by kldload, vnet propagation will be done after registering sysctls, then previous propagation is not needed.

MFC after: 3 weeks

Test Plan

Load modules that have sysctl knobs flagged with both CTLFLAG_TUN and CTLFLAG_VNET.

# echo "net.link.bridge.log_stp=1" >> /boot/loader.conf
# echo 'if_bridge_load="YES"' >> /boot/loader.conf
# reboot
# sysctl net.link.bridge.log_stp
net.link.bridge.log_stp: 1
# kldunload if_bridge
# kldload if_bridge
# sysctl net.link.bridge.log_stp
net.link.bridge.log_stp: 1

[kib]

You need to run this through the group of people maintaining VNET.

[zlei]

This change depends on D39638 .