lib/packet/packet_utils: optimize checksum
lib/packet/packet_utils: optimize checksum

builtin function, sum, is much faster than for loop.
The result on my machine is as follows

> def main():
>     from timeit import timeit
>     data = bytearray().zfill(1500)
>     print 'new=', timeit(lambda : checksum(data), number=1000)
>     print 'old=', timeit(lambda : checksum_old(data), number=1000)
>
> new= 0.00800108909607
> old= 0.266770124435

Signed-off-by: Isaku Yamahata <4b3abd66323411eeceff090ab3754dd766fc4b25@valinux.co.jp>
Signed-off-by: FUJITA Tomonori <93dac1fe9c4b2a3957982200319981492ad4976e@lab.ntt.co.jp>

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