sunrpc: add an "info" file for the dummy gssd pipe

rpc.gssd expects to see an "info" file in each clntXX dir. Since adding
the dummy gssd pipe, users that run rpc.gssd see a lot of these messages
spamming the logs:

    rpc.gssd[508]: ERROR: can't open /var/lib/nfs/rpc_pipefs/gssd/clntXX/info: No such file or directory
    rpc.gssd[508]: ERROR: failed to read service info

Add a dummy gssd/clntXX/info file to help silence these messages.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>