util/disk_cache: delete more cache items in one go when full

Currently the cache just deletes enough items when the cache is full to make room for the new item being stored. This hasn't been too much of a problem in practice but for things like running piglit where we have thousands of unique shaders and all threads being utilised we end up with a pretty big bottle neck.

With this change rather than just brute forcing our way to having enough room for the new item, we instead grab 10% of the least recently used items in the random directory we chose and delete them all. This should only be around 0.04% of total cache items but should hopefully releave some of the pressure on system calls like fstatat().