tcp_cubic: use memset and offsetof init

In bictcp_reset(), use memset and offsetof instead of = 0.

Signed-off-by: Yejune Deng <yejune.deng@gmail.com>
Link: https://lore.kernel.org/r/1610597696-128610-1-git-send-email-yejune.deng@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>