Previously we used to just set FULL_BACKREF if we couldn't lookup an extent info
for an extent. Now we just bail out if we can't lookup the extent info, which
is less than good since fsck is supposed to fix these very problems. So instead
figure out the flag we are supposed to use and pass that along instead. This
patch also provides a test image to test this functionality. Thanks,
Signed-off-by: Josef Bacik <jbacik@fb.com>
