SUNRPC: rpcbind should never reset the port to the value '0'

If we already had a valid port number for the RPC service, then we
should not allow the rpcbind client to set it to the invalid value '0'.

Reviewed-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: Benjamin Coddington <bcodding@redhat.com>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>