mptcp: preserve const qualifier in mptcp_sk()

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

We need to change few things to avoid build errors:

mptcp_set_datafin_timeout() and mptcp_rtx_head() have to accept
non-const sk pointers.

@msk local variable in mptcp_pending_tail() must be const.

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