tcp: preserve const qualifier in tcp_sk()

We can change tcp_sk() to propagate its argument const qualifier,
thanks to container_of_const().

We have two places where a const sock pointer has to be upgraded
to a write one. We have been using const qualifier for lockless
listeners to clearly identify points where writes could happen.

Add tcp_sk_rw() helper to better document these.

tcp_inbound_md5_hash(), __tcp_grow_window(), tcp_reset_check()
and tcp_rack_reo_wnd() get an additional const qualififer
for their @tp local variables.

smc_check_reset_syn_req() also needs a similar change.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: David S. Miller <davem@davemloft.net>