mke2fs.c (main): Zap the second sector of the disk, along with

mke2fs.c (main): Zap the second sector of the disk, along with
        any sectors in the same filesystem block after the superblock.
        The latter will remove (for example) swapspace signatures
        on 4kB+ blocksize filesystems.  Also when zeroing the "end"
        of the filesystem don't actually zero the start of a very
        small device (less than 128kB).