InnoDB linear read ahead had better be logical

MDEV-30986 was originally filed due to a change that was made in MDEV-24854: Starting with MariaDB Server 10.6, InnoDB disables the file system cache by default (innodb_flush_method=O_DIRECT).

While disabling the file system cache does improve write performance, it can hurt the performance of those read workloads that cannot be satisfied directly by the InnoDB buffer pool.

In MDEV-30986 it was tested on several types of storage and operating system versions that when the data needs to be loaded into the InnoDB buffer pool, it is faster when the file system cache of the operating system is used. This is the case also when the cache is initially empty. This suggests that the InnoDB read-ahead mechanism could be better.

In key range scans or table scans, it would seem to make sense to post read-ahead requests for the index leaf pages when the level right above the leaf is reached. At that point, we would know which pages will have to be accessed by the query. Possibly, it would help to issue a single larger read instead of several single-page requests (MDEV-11378).