NFS: remove revoked delegation from server's delegation list

After the delegation is returned to the NFS server remove it
from the server's delegations list to reduce the time it takes
to scan this list.

Network trace captured while running the below script shows the
time taken to service the CB_RECALL increases gradually due to
the overhead of traversing the delegation list in
nfs_delegation_find_inode_server.

The NFS server in this test is a Solaris server which issues
CB_RECALL when receiving the all-zero stateid in the SETATTR.

mount=/mnt/data
for i in $(seq 1 20)
do
   echo $i
   mkdir $mount/testtarfile$i
   time  tar -C $mount/testtarfile$i -xf 5000_files.tar
done
Signed-off-by: Dai Ngo <dai.ngo@oracle.com>
Reviewed-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Signed-off-by: Anna Schumaker <anna.schumaker@oracle.com>

fs/nfs/delegation.c

@@ -1001,6 +1001,11 @@ void nfs_delegation_mark_returned(struct inode *inode,
 	}
 
 	nfs_mark_delegation_revoked(delegation);
+	clear_bit(NFS_DELEGATION_RETURNING, &delegation->flags);
+	spin_unlock(&delegation->lock);
+	if (nfs_detach_delegation(NFS_I(inode), delegation, NFS_SERVER(inode)))
+		nfs_put_delegation(delegation);
+	goto out_rcu_unlock;
 
 out_clear_returning:
 	clear_bit(NFS_DELEGATION_RETURNING, &delegation->flags);