NFSv4: Clean up initialisation of uniquified client id strings

When the user sets a uniquifier, then ensure we copy the string
so that calls to strlen() etc are atomic with calls to snprintf().

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