snapshot: change 'struct SnapShotPage' to a simple typedef

snapshot: change 'struct SnapShotPage' to a simple typedef

The struct SnapShotPage was being used improperly; it was accessed as
simply an array of data, instead of explicitly accessing its member
variable (which itself was an array). So just make this more
straightforward as a typedef'd array.

This also corrects a pointer cast to use (void *) when appropriate.