NFS: return delegations from the end of a LRU when over the watermark

NFS: return delegations from the end of a LRU when over the watermark

Directly returning delegations on close when over the watermark is
rather suboptimal as these delegations are much more likely to be reused
than those that have been unused for a long time.  Switch to returning
unused delegations from a new LRU list when we are above the threshold and
there are reclaimable delegations instead.

Pass over referenced delegations during the first pass to give delegations
that aren't in active used by frequently used for stat() or similar another
chance to not be instantly reclaimed.  This scheme works the same as the
referenced flags in the VFS inode and dentry caches.

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