Clean up fsck_ffs error message output.

When fsck_ffs is creating a lost+found directory it must allocate
an inode and a filesystem block. If it encounters a cylinder group
with a bad check hash, it complains twice: once for the inode and
again for the filesystem block.

This change suppresses the second complaint.

Reported by: Chuck Silvers
Tested by: Chuck Silvers
MFC after: 1 week
Sponsored by: Netflix