e2fsck: workaround for old extents tests

e2fsck: workaround for old extents tests

The e2fsck_ext2fs_extent_get() part of this patch is a workaround
to handle problems with old Lustre extents patches that didn't
clear the ee_start_hi or ei_leaf_hi fields.

That has been fixed for long time and could be removed as soon
as the f_extent_* tests are fixed to clear these _hi fields.
Otherwise the extents are all marked as corrupt and it ruins those
tests value.

Signed-off-by: Andreas Dilger <adilger@whamcloud.com>