selftests/net: Add SHA256 computation over data sent in tcp_mmap

Add option to compute and send SHA256 over data sent (-i).

This is to ensure the correctness of data received.
Data is randomly populated from /dev/urandom.

Tested:
./tcp_mmap -s -z -i
./tcp_mmap -z -H $ADDR -i
SHA256 is correct

./tcp_mmap -s -i
./tcp_mmap -H $ADDR -i
SHA256 is correct

Signed-off-by: Coco Li <lixiaoyan@google.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://lore.kernel.org/r/20230321081202.2370275-2-lixiaoyan@google.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>