We need to zero out the private data area when application switches
connection to different algorithm (TCP_CONGESTION setsockopt).

When congestion ops get assigned at connect time everything is already
zeroed because sk_alloc uses GFP_ZERO flag.  But in the setsockopt case
this contains whatever previous cc placed there.
Signed-off-by: NFlorian Westphal <fw@strlen.de>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>