Don't keep hand coded offset caluclations and replace it with
container_of(). It should be type safer and a bit less confusing.
It also makes it with a macro instead of inline function to preserve
constness, which was previously casted out like in case of
tcp_v6_send_synack().
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com> Reviewed-by: Simon Horman <simon.horman@corigine.com> Signed-off-by: David S. Miller <davem@davemloft.net>