iotests/026: Test EIO on preallocated zero cluster

Test what happens when writing data to a preallocated zero cluster, but
the data write fails.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-Id: <20200225143130.111267-3-mreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>