nfsd: rename nfsd_last_thread() to nfsd_destroy_serv()

As this function now destroys the svc_serv, this is a better name.

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