nfs_fha_new: Fix nfs_fha_new so that sysctls work in prisons

The sysctls were in "struct fha_params", making it impractical
to vnet them. This patch moves the sysctls out of "struct fha_params"
and vnet's them, so that they are available in vnet prisons
running nfsd(8). It also avoids the IS_DEFAULT_VNET() macros
in the VNET_SYSINIT() function.

Reviewed by: glebius
MFC after: 3 months
Differential Revision: https://reviews.freebsd.org/D38748