SUNRPC: Replace clnt->cl_principal

The clnt->cl_principal is being used exclusively to store the service
target name for RPCSEC_GSS/krb5 callbacks. Replace it with something that
is stored only in the RPCSEC_GSS-specific code.

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>