buf_read_page() should return a buffer-fixed block

Every caller of buf_read_page() is expecting to gain access to the block. If that function returned a buffer-fixed block descriptor, we could avoid some busy work, such as a redundant lookup of buf_pool.page_hash.

This change was initially part of the 11.4 development version of MDEV-34431, but there are some issues with it, apparently some hangs due to not released buffer-fix under some special circumstances. In my own performance testing yesterday, I observed a 4% performance improvement in an I/O bound read-write workload where the active working set does not fit in the buffer pool.