NFSv4: Be less aggressive about returning delegations for open files

Currently, if the application that holds the file open isn't doing
I/O, we may end up returning the delegation. This means that we can
no longer cache the file as aggressively, and often also that we
multiply the state that both the server and the client needs to track.

This patch adds a check for open files to the routine that scans
for delegations that are unreferenced.

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