tcp: add ability to set a timestamp offset

If a TCP socket will get live-migrated from one box to another the
timestamps (which are typically ON) will get screwed up -- the new
kernel will generate TS values that has nothing to do with what they
were on dump. The solution is to yet again fix the kernel and put a
"timestamp offset" on a socket.

Signed-off-by: Andrey Vagin <avagin@openvz.org>