nfsd: recover: constify nfsd4_client_tracking_ops structures

The nfsd4_client_tracking_ops structures are never modified, so declare
them as const.

Done with the help of Coccinelle.

Signed-off-by: Julia Lawall <Julia.Lawall@lip6.fr>
Reviewed-by: Jeff Layton <jlayton@poochiereds.net>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>