If a locking level wasn't specified locking used to stop. This means
that if one, for example, didn't specify anything at the LEVEL_INSTANCE
level, no locks at the LEVEL_NODE level were acquired either. With this
patch we force _LockAndExecLU to be called for all existing levels, and
break the recursion if the level doesn't exist in locking.LEVELS.

Reviewed-by: imsnah