Tests with 4096-byte sector size confirm that it is
safe to use O_DIRECT with page_compressed tables.
That had been disabled on Linux, in an attempt to fix MDEV-21584
which had been filed for the O_DIRECT problems earlier.

The fil_node_t::block_size was being set mostly correctly until
commit 10dd290b (MDEV-17380)
introduced a regression in MariaDB Server 10.4.4.

fil_node_t::read_page0(): Initialize fil_node_t::block_size.
This will probably make similar code in fil_space_extend_must_retry()
redundant, but we play it safe and will not remove that code.

Thanks to Vladislav Vaintroub for testing this on Microsoft Windows
using an old-fashioned rotational hard disk with 4KiB sector size.

Reviewed by: Vladislav Vaintroub