NFS: make nfs_mark_return_unreferenced_delegations less aggressive

NFS: make nfs_mark_return_unreferenced_delegations less aggressive

Currently nfs_mark_return_unreferenced_delegations marks all open but
not referenced delegations (i.e., those were found by a previous pass)
as return on close, which means that we'll return them on close without
a way out.

Replace this with only iterating delegations that are on the LRU list,
and avoid delegations that are in use by an open files to avoid this.

Delegations that were never referenced while open still are be prime
candidates for return from the LRU if the number of delegations is over
the watermark, or otherwise will be returned by the next
nfs_mark_return_unreferenced_delegations pass after they are closed.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Anna Schumaker <anna.schumaker@oracle.com>