madvise(frame, srv_page_size, MADV_FREE) is causing a performance regression

In MDEV-25341 (MariaDB Server 10.11.0), some calls to madvise() with MADV_FREE were added.

If the call in buf_LRU_block_free_non_file_page() is committed out, the throughput in an I/O bound HammerDB benchmark would improve by 6% to 18%, depending on the CPU microarchitecture.

It could be best to revert this code for now. A better solution might be doable as part of a larger refactoring (MDEV-25340, MDEV-29445). Instead of invoking madvise(MADV_FREE) on individual buffer pages, we might invoke it on larger chunks at a time. There could also be an explicit SQL statement for defragmenting or freeing the buffer pool.