- Only set up pipefs entries for those RPC services that actually
   need them (for the moment NFS only). Portmap, lockd,... shouldn't
   need to make upcalls.

 - Add in missing semaphore in rpc_populate().

 - Make inode/dentry variable names in rpc_depopulate/rpc_populate
   more consistent w.r.t other functions in rpc_pipe.c

 - Call shrink_dcache_parent() in order to clean up child entries
   before we rmdir().