Fix backward compatibility with UFS1 filesystems created before June 2002

See the Pahabricator review for more details.

Reviewed-by: kib
Tested-by: Peter Holm
Differential-Revision: https://reviews.freebsd.org/D48472
(cherry picked from commit 661ca921e8cd56b17fc6615bc7e596e56e0e7c31)