NFSv4: Use the net namespace uniquifier if it is set

If a container sets a net namespace specific uniquifier, then use that
in the setclientid/exchangeid process.

Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>