xfstests 285: Fix test for ext4 in some configurations

xfstests 285: Fix test for ext4 in some configurations

In some configurations (e.g. 1 KB block size), ext4 can decide it is
better to zero out several blocks rather than splitting unwritten
extent. This changes results SEEK_HOLE / SEEK_DATA returns and thus the
test fails. Fix the problem by disabling the feature for this test.

Signed-off-by: Jan Kara <jack@suse.cz>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Rich Johnston <rjohnston@sgi.com>