SUNRPC: Clean up rpc_pipefs.

Add a dentry_ops with a d_delete() method in order to ensure that dentries
are removed as soon as the last reference is gone.

Clean up rpc_depopulate() so that it only removes files that were created
via rpc_populate().

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