Update fsdb(8) to reflect new structure of fsck_ffs(8).

The cleanup of fsck_ffs(8) in commit c0bfa109b942659f6 broke fsdb(8).
This commit adds the one-line update needed in fsdb(8) to make it
work with the new fsck_ffs(8) structure.

Reported by: Chuck Silvers
Tested by: Chuck Silvers
MFC after: 3 days