fix: get_num_blocks_touched logic

[AlpinDale]

If prefix caching is enabled, and the blocks are swapped out, the left-over caches will be in the evictor group if there are no references. In the meantime, the cache blocks will be evicted, for example, 2 out of 10 cached blocks. Let's say the remaining 8 blocks in the evictor are the last available blocks in the allocator, and the sequence wants to swapped in, the free blocks will count as 8. The touched blocks is calculated as 2, which is incorrect because the cache is not shared by any other blocks. In this case the correct touched blocks should be 10, and can't swap in. In the regular case, if the lookahead slots are more than the free slots, it first need to occupy the block containing the free slots, then plus the remaining required blocks.