It is illegal to change arbitrary fields in skb_shared_info if the
skb is cloned.
Before calling skb_zcopy_clear() we need to ensure this rule,
therefore we need to move the test from sk_stream_alloc_skb()
to sk_wmem_free_skb()
Fixes: 4f661542a402 ("tcp: fix zerocopy and notsent_lowat issues") Signed-off-by: Eric Dumazet <edumazet@google.com> Diagnosed-by: Willem de Bruijn <willemb@google.com> Signed-off-by: David S. Miller <davem@davemloft.net>