Clear nilfs2 secondary super block (#688882)

Wipefs only clears the nilfs2 file system signature from the primary
super block at the start of the partition.  This is enough for blkid to
no longer detect the file system.  However parted (>= 2.4 with nilfs2
support) and therefore GParted still detect the file system using the
secondary super block at the end of the partition.

    # mkfs.nilfs2 /dev/sda12
    # wipefs -a /dev/sda12
    2 bytes were erased at offset 0x00000406 (nilfs2): 34 34
    # blkid /dev/sda12
    # parted /dev/sda12 print
    Model: Unknown (unknown)
    Disk /dev/sda12: 1074MB
    Sector size (logical/physical): 512B/512B
    Partition Table: loop
    Disk Flags:

    Number  Start  End     Size    File system  Flags
     1      0.00B  1074MB  1074MB  nilfs2

Overwrite the nilfs2 secondary super block at the end of the partition
with 4K block of zeros.  Always do this just in case the partition
contains a nilfs secondary super block but is detected as a different
file system.  Such an example was documented in an earlier commit
message.

Bug #688882 - Improve clearing of file system signatures