mkfs: don't crash on dswidth overflow

mkfs: don't crash on dswidth overflow

I ran mkfs.xfs -d su=1048576,sw=$((18 * 1048576)), forgetting that sw
takes a multiple of su (unlike swidth which takes any space unit).  I
was surprised when we hit a floating point exception, which I traced
back to an integer overflow when we calculate swidth from dsw.

So, do the 64-bit multiplication so we can detect the overflow and
complain about it.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>