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fix(virtio): sync disk data on virtio-blk FLUSH

VirtioMio::handle_queue() previously handled BlkRequest::Flush by
calling <&File as Write>::flush(), which is a no-op in the Rust standard
library and never synchronizes data to disk.

Per VirtIO 1.4 Sec 5.2.6.2, a block device backed by persistent storage
must ensure writes are committed to it before reporting completion of a
VIRTIO_BLK_T_FLUSH request. Call File::sync_data() instead.

A flush only needs the written data and the metadata required to read
it back, such as the file size and block allocation, not timestamps.
So also set IORING_FSYNC_DATASYNC on the fsync in the io_uring worker,
which makes both workers use fdatasync(2) on Linux. On macOS,
sync_data() issues F_FULLFSYNC, the same as sync_all().

Fixes: 9a3857f84dc2 ("fix(blk): flush disk file on FLUSH requests")
Signed-off-by: Changyuan Lyu <changyuanl@google.com>

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