lockd: add a /proc/fs/lockd/nlm_end_grace file

Add a new procfile that will allow a (privileged) userland process to
end the NLM grace period early. The basic idea here will be to have
sm-notify write to this file, if it sent out no NOTIFY requests when
it runs. In that situation, we can generally expect that there will be
no reclaim requests so the grace period can be lifted early.

Signed-off-by: Jeff Layton <jlayton@primarydata.com>