lockd: add netlink control interface

The legacy rpc.nfsd tool will set the nlm_grace_period if the NFSv4
grace period is set. nfsdctl is missing this functionality, so add a new
netlink control interface for lockd that it can use. For now, it only
allows setting the grace period, and the tcp and udp listener ports.

lockd currently uses module parameters and sysctls for configuration, so
all of its settings are global. With this change, lockd now tracks these
values on a per-net-ns basis. It will only fall back to using the global
values if any of them are 0.

Finally, as a backward compatibility measure, if updating the nlm
settings in the init_net namespace, also update the legacy global
values to match.

Link: https://issues.redhat.com/browse/RHEL-71698


Signed-off-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>