NFSv4: Clean up nfs_client_return_marked_delegations()

Convert it to use the nfs_client_for_each_server() helper, and
make it more efficient by skipping delegations for inodes we
know are in the process of being freed. Also improve the efficiency
of the cursor by skipping delegations that are being freed.

Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>