NFS: Set default port for NFSv4, with support for AF_INET6

Create a helper function to set the default NFS port for NFSv4 mount
points.  The helper supports both AF_INET and AF_INET6 family addresses.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>