As btrfs showed, a partial read of data in AIO /O_DIRECT circumstances can
really confuse MariaDB.

Filipe Manana (SuSE)[1] showed how database programmers can assume
O_DIRECT is all or nothing.

While a fix was done in the kernel side, we can do better in our code by
requesting that the rest of the block be read/written synchronously if
we do only get a partial read/write.

Per the APIs, a partial read/write can occur before an error, so
reattempting the request will leave the caller with a concrete error to
handle.

[1] https://lore.kernel.org/linux-btrfs/CABVffENfbsC6HjGbskRZGR2NvxbnQi17gAuW65eOM+QRzsr8Bg@mail.gmail.com/T/#mb2738e675e48e0e0778a2e8d1537dec5ec0d3d3a

Also spell synchronously correctly in other files.