stcp: Use memset_after() to zero sctp_stream_out_ext

Use memset_after() helper to simplify the code, there is no functional
change in this patch.

Signed-off-by: Xiu Jianfeng <xiujianfeng@huawei.com>
Link: https://lore.kernel.org/r/20220519062932.249926-1-xiujianfeng@huawei.com


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