Don't close and reopen all pipes on every DNOTIFY signal.

	From: Vince Busam <vbusam@google.com>
	Signed-off-by: Kevin Coffman <kwc@citi.umich.edu>
	Don't unnecessarily close and re-open all pipes after every DNOTIFY
	signal.  These unnecessary closes were triggering a kernel Oops.
	Original patch modified to correct segfault when unmounting last
	NFSv4 mount.