SUNRPC: Convert rpc_pipefs to use the generic filesystem notification hooks

This will allow rpc.gssd to use inotify instead of dnotify in order to
locate new rpc upcall pipes.

This also requires the exporting of __audit_inode_child(), which is used by
fsnotify_create() and fsnotify_mkdir(). Ccing David Woodhouse.

Cc: David Woodhouse <dwmw2@infradead.org>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>