Cleanup TTMs delayed delete handling

This series of patches cleans up TTMs delayed delete handling.

The core of the new handling is that we new only have a single reference counter instead of two and use kref_get_unless_zero() to grab BOs from the LRU during eviction.

This reduces the overhead of LRU moves and allows us to properly individualize the BOs reservation object during deletion to allow adding BOs for clearing memory, unmapping page tables etc..

Please review and comment, Christian.