selftests: net: change fprintf format specifiers

`cur64`, `start64` and `ts_delta` are int64_t. Change format
specifiers in fprintf from `"%lu"` to `"%" PRId64` to adapt
to 32-bit and 64-bit systems.

Signed-off-by: Guo Zhengkui <guozhengkui@vivo.com>
Link: https://lore.kernel.org/r/20220319073730.5235-1-guozhengkui@vivo.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>