file-posix: Make bdrv_flush() failure permanent without O_DIRECT

file-posix: Make bdrv_flush() failure permanent without O_DIRECT

Success for bdrv_flush() means that all previously written data is safe
on disk. For fdatasync(), the best semantics we can hope for on Linux
(without O_DIRECT) is that all data that was written since the last call
was successfully written back. Therefore, and because we can't redo all
writes after a flush failure, we have to give up after a single
fdatasync() failure. After this failure, we would never be able to make
the promise that a successful bdrv_flush() makes.

Signed-off-by: Kevin Wolf <[email protected]>
Message-id: 20170322210005[email protected]
Reviewed-by: Fam Zheng <[email protected]>
Reviewed-by: Eric Blake <[email protected]>
Reviewed-by: Stefan Hajnoczi <[email protected]>
Signed-off-by: Max Reitz <[email protected]>