Have fsck_ffs(8) properly correct superblock check-hash failures.

Part of the problem was that fsck_ffs would read the superblock
multiple times complaining and repairing the superblock check hash
each time and then at the end failing to write out the superblock
with the corrected check hash. This fix reads the superblock just
once and if the check hash is corrected ensures that the fixed
superblock gets written.

Tested by: Peter Holm
PR: 245916
MFC after: 1 week
Sponsored by: Netflix