mkfs.xfs: don't call blkid_get_topology on existing regular files

mkfs.xfs: don't call blkid_get_topology on existing regular files

If we encounter a target that's really a regular file,
even without "-d file..." on the cmdline, call
platform_findsizes() instead of blkid_get_topology to
try to discover the "sector size" via the fsgeom() call.

Otherwise mkfs.xfs will try to do direct IO with a default
512 sector size, and if the underlying file has different
DIO requirements, mkfs will fail.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>