generic: test data corruption on ext4 caused by written/delayed extent

generic: test data corruption on ext4 caused by written/delayed extent

This test exercises the problem with unwritten and delayed extents
in ext4 extent status tree where we might in some cases lose a block
worth of data. Even though this was a ext4 specific problem the
reproducer can be easily run on any file system so let's do that just
in case.

This test exercises the problem fixed in kernel with commit
"ext4: Fix data corruption caused by unwritten and delayed extents"

Signed-off-by: Lukas Czerner <lczerner@redhat.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>