dd->total is unsigned so it won't do any good to check for negative
value after subtracting instead of that we should check if the
subtracted value is bigger than him

This was partially found by using a static code analysis program
called cppcheck.
Signed-off-by: NAsaf Vertz <asaf.vertz@tandemg.com>
Signed-off-by: NHerbert Xu <herbert@gondor.apana.org.au>