SUNRPC: remove RPC client pipefs dentries after unregister

Without this patch we have races:

rpc_fill_super				rpc_free_client
rpc_pipefs_event(MOUNT)			rpc_remove_pipedir
spin_lock(&rpc_client_lock);
rpc_setup_pipedir_sb
spin_unlock(&rpc_client_lock);
					spin_lock(&rpc_client_lock);
					(remove from list)
					spin_unlock(&rpc_client_lock);
					MEAMORY LEAKED

Signed-off-by: Stanislav Kinsbursky <skinsbursky@parallels.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>