RPCSEC_GSS: Clean up upcall message allocation

Optimise away gss_encode_msg: we don't need to look up the pipe
version a second time.

Save the gss target name in struct gss_auth. It is a property of the
auth cache itself, and doesn't really belong in the rpc_client.

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