lockd: move svc_exit_thread() into the thread

The normal place to call svc_exit_thread() is from the thread itself
just before it exists.
Do this for lockd.

This means that nlmsvc_rqst is not used out side of lockd_start_svc(),
so it can be made local to that function, and renamed to 'rqst'.

Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>