NFSD: SETCLIENTID_CONFIRM returns NFS4ERR_CLID_INUSE too often

According to RFC 3530bis, the only items SETCLIENTID_CONFIRM processing
should be concerned with is the clientid, clientid verifier, and
principal.  The client's IP address is not supposed to be interesting.

And, NFS4ERR_CLID_INUSE is meant only for principal mismatches.

I triggered this logic with a prototype UCS client -- one that
uses the same nfs_client_id4 string for all servers.  The client
mounted our server via its IPv4, then via its IPv6 address.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>