nfsd: Add NFSD_VNET() macros to nfs_fha_new.c

Although it is not 100% obvious if the affinity threads
lists need to be vnet'd when nfsd runs in a prison, I
think it is necessary for the unusual case where the
same file system is exported in multiple prisons.
For this case, the affinity code might try to assign
the RPC to a svc thread that runs in a different prison.
Also, it makes sense to vnet them, since there are
separate svc threads for each prison running nfsd(8).

This patch adds the macros for vnet'ng to nfs_fha_new.c.
The macros are still null, so the only semantics change
is malloc'ng the fha_param structure.

MFC after: 3 months