SUNRPC: One more clean up for rpc_create_client_dir()

In order to allow rpc_pipefs to create directories with different types of
subtrees, it is useful to allow the caller to customise the subtree filling
process.
In order to do so, we separate out the parts which are specific to making
an RPC client directory, and put them in a separate helper, then we convert
the process of filling the directory contents into a callback.

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