[RFC] TCP ISN access from userlevel

Hello!

Expansion of Linux Kernel API for getting from userlevel TCP ISN by
socket descriptor. Would such improvement deserve merge? The use case
is to differ hosts behind NAT. The general plan of changes:

1. copy ISN into the tcp_sock structure when the TCP session is created;
2. add ioctl for getting ISN;
   or
2. copy that value into the tcp_info structure returned by getsockopt().

I see that 'snt_isn' and 'rcv_isn' are stored in 'tcp_request_sock'.
Is 'tcp_request_sock' have the lifetime of 'tcp_sock'? I.e. can it be
always accessed via 'fastopen_rsk'?

Please, CC me on reply.