Make nfs_fha sysctls per vnet and get rid of IS_DEFAULT_VNET()

[rmacklem]

Summary

Instead of only doing the tuning sysctls for prison0
by using IS_DEFAULT_VNET(), make them per-vnet.
This gets rid of the IS_DEFAULT_VNET() calls in
the initialization function.

Test Plan

Tested on a system that is running nfsd(8) in
vnet prisons ad well as prison0.

[glebius]

Looks correct and better, thanks! Disclaimer: didn't test.