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 <kwolf@redhat.com>
Message-id: 20170322210005.16533-1-kwolf@redhat.com
Reviewed-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>