If the block size is 64k, the first alternate ffs1 superblock ends up
in a location first looked at by dumpfs.
fsck_ffs(8) (see setup.c) and ffs_mountfs() in
sys/ufs/ffs/ffs_vfsops.c have protection against that case, since we
really want the primary superblock, that's the one that is actually
updated when a fs is used.