tcp: Sanitize CMSG flags and reserved args in tcp_zerocopy_receive.

Explicitly define reserved field and require it and any subsequent
fields to be zero-valued for now. Additionally, limit the valid CMSG
flags that tcp_zerocopy_receive accepts.

Fixes: 7eeba170

 ("tcp: Add receive timestamp support for receive zerocopy.")
Signed-off-by: Arjun Roy <arjunroy@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Soheil Hassas Yeganeh <soheil@google.com>
Suggested-by: David Ahern <dsahern@gmail.com>
Suggested-by: Leon Romanovsky <leon@kernel.org>
Suggested-by: Jakub Kicinski <kuba@kernel.org>
Acked-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>