packet_utils: Remove checksum() side-effect
packet_utils: Remove checksum() side-effect

checksum() was modifying argument when it is bytearray.  Make sure
checksum() doesn't modify its argument.

Signed-off-by: IWAMOTO Toshihiro <8e41ae29694291a01b8efca6b4a0005a87ddb93e@valinux.co.jp>
Signed-off-by: FUJITA Tomonori <93dac1fe9c4b2a3957982200319981492ad4976e@lab.ntt.co.jp>

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