tcp_nv: use do_div() instead of expensive div64_u64()

Average RTT is 32-bit thus full 64-bit division is redundant.

Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Suggested-by: Stephen Hemminger <stephen@networkplumber.org>
Suggested-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>