Re-organize the NFS file handle affinity code for the NFS server.

The file handle affinity code was configured to be used by both the
old and new NFS servers. This no longer makes sense, since there is
only one NFS server.
This patch copies a majority of the code in sys/nfs/nfs_fha.c and
sys/nfs/nfs_fha.h into sys/fs/nfsserver/nfs_fha_new.c and
sys/fs/nfsserver/nfs_fha_new.h, so that the files in sys/nfs can be
deleted. The code is simplified by deleting the function callback pointers
used to call functions in either the old or new NFS server and they were
replaced by calls to the functions.

As well as a cleanup, this re-organization simplifies the changes
required for handling of external page mbufs, which is required for KERN_TLS.

This patch should not result in a semantic change to file handle affinity.