Stop over rounding up of signature zeroing lengths (#756829)

Following the previous commit "Add erasing of SWRaid metadata 0.90 and
1.0 super blocks (#756829)" signature zeroing specified to write 4 KiB
of zeros at position end - 64 KiB, aligned to 64 KiB.  Example operation
details from formatting a 1 GiB partition to cleared:

    Format /dev/sdb8 as cleared
    + calibrate /dev/sdb8
    + clear old file system signatures in /dev/sdb8
      + write 68.00 KiB of zeros as byte offset 0
      + wite 4.00 KiB of zeros at byte offset 67108864
      + wite 64.00 KiB of zeros at byte offset 1073676288
      + write 8.00 KiB of zeros at byte offset 1073733632
      + flush operating system cache of /dev/sdb

However it actually wrote 64 KiB.  This is because the rounding /
alignment was also applied to the zeroing length.  Before this commit
rounding / alignment was always less than or equal to the length so this
wasn't seen before.  Instead just apply device sector size rounding up
to the length.

Bug 756829 - SWRaid member detection enhancements