ext4: test migration to non-extent based file format

ext4: test migration to non-extent based file format

On unpatched kernel, converting file with a hole at the beginning to
non-extent based format results in ext4 i_blocks corruption. Add a new
regression test case for it.

These two commits fixed the corruption:
ext4: be more strict when migrating to non-extent based file
ext4: correctly migrate a file with a hole at the beginning

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