Spinning on buf_pool.page_hash is wasting CPU cycles

This came up in MDEV-26769 and was also pointed out by krunalbauskar.

When I introduced a new type of rw-lock for buf_pool.page_hash in MDEV-22871, I thought that conflicts would be extremely rare and the critical sections very short. Therefore, only a spinloop was implemented. In 10.6, we could easily use an alternative that was introduced in MDEV-24167.

Only on systems for which we have not implemented a futex interface (see also MDEV-26476), we’d better keep the spinloop, because small size of the rw-lock is important.