align nfsdumpstate column output

There are scenarios where an NFS client will mount an NFSv4 export
without specifying a callback address.

When running nfsdumpstate under this circumstance, the column output is
shifted incorrectly which places the "ClientID" value underneath the
"Clientaddr" column.

This diff is a small cosmetic change that prints a blank in the
"Clientaddr" column and ensures the data for the columns are aligned
appropriately.

Submitted by: Caleb St. John
Reviewed by: sef (previous version)
MFC after: 3 days
Sponsored by: iXsystems, Inc.
Differential Revision: https://reviews.freebsd.org/D18958