TAKE - fix previous mkfs.xfs fix

[Eric Sandeen <sandeen@xxxxxxx>]

My previous TAKE, which zeroed out other FS signatures, broke if you tried to
make a filesystem on a file (as opposed to a block device).  This fixes it.

Date:  Mon Jan 14 10:31:28 PST 2002
Workarea:  stout.americas.sgi.com:/localhome/eric/2.4.x-xfs/workarea-reallyclean

The following file(s) were checked into:
  bonnie.engr.sgi.com:/isms/slinx/2.4.x-xfs


Modid:  xfs-cmds:slinx:109530a
cmd/xfsprogs/mkfs/xfs_mkfs.c - 1.21
        - Don't try to zero last 64k on the device if the filesystem is in a 
regular
          file.