NFS: switch NFSROOT to use new rpcbind client

It is arguable whether NFSROOT will support IPv6, and thus whether
rpcb_getport_external needs to support rpcbind versions greater than 2.

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