lockdep: Reduce matrices usage range to minimum

lockdep: Reduce matrices usage range to minimum

Introduce current_maxid variable which holds highest lock ID. Using that,
don't attempt to check/reset parts of "follows" and "follows_bt" matrices
that are not used yet. This improves performance by reducing CPU usage
(particularly by lockdep_unregister() function), this also reduces RSS
memory usage by quite a bit, as many allocated pages are not actually
touched anymore. Improve CPU usage a bit more, by using memset() to
reset "follows" members instead of iterating byte-by-byte.
Finally, use push_front instead of push_back when unregistering, so
unregistered IDs will be recycled.

Signed-off-by: Piotr Dałek <git@predictor.org.pl>