gssd: add upcall support for callback authentication

Change the processing so that all subdirectories within the rpc_pipefs
directory are treated equally.  Any "clnt" directories that show up
within any of them are processed.  (As suggested by Bruce Fields.)

Note that the callback authentication will create a new "nfs4d_cb"
subdirectory.  Only new kernels (2.6.29) will create this new directory.
(The need for this directory will go away with NFSv4.1 where the
callback can be done on the same connection as the fore-channel.)

Signed-off-by: Kevin Coffman <kwc@citi.umich.edu>
Signed-off-by: Steve Dickson <steved@redhat.com>