selftests: net: Use size_t and ssize_t for counting file size

Use size_t and ssize_t correctly for counting send file size
instead of unsigned long and long, because long is 32bit on
32bit arch, which is not enough for counting long file size (>4GB).

Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Eric Dumazet <edumazet@google.com>
Cc: David S. Miller <davem@davemloft.net>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>