nfsd: use static memory for per-vnet struct fha_params

[glebius]

Summary

Use VIMAGE framework to instantiate per-VNET static structures. The
dynamic sysctl creation seems to be unneeded from the very beginning
of this module, use static initialization for them, too.

[rmacklem]

Due to vnet space constraints, I would prefer to
leave the fha_param structure malloc'd.

I will take the sysctls out of it and try and vnet
them. These sysctls are rarely used, so having
them in prison0 only is not that serious a problem,
but I will try and fix it.

Btw, with everything malloc'd, the vnet footprint
for the NFS server and all related stuff (krpc, kgssapi)
is about 500 bytes. Since "struct fha_param" is 10120bytes,
making it static will probably make "kldload nfsd.ko" fail.

[glebius]

See D38748