socket: values of snd and rcv buffer should be divide into two

This values will be doubled in kernel to account for
"struct sk_buff" etc. overhead.

Currently criu restores snd and rcv buffer limits incorrectly,
they become bigger on each iteration.
$ ../crit show dump/zdtm/live/static/socket-tcp/6299/1/inetsk.img | grep buf
                "so_sndbuf": 2626560,
                "so_rcvbuf": 1060720,
                "so_sndbuf": 16384,
                "so_rcvbuf": 87380,
$ ../crit show dump/zdtm/live/static/socket-tcp/6299/2/inetsk.img | grep buf
                "so_sndbuf": 5253120,
                "so_rcvbuf": 2121440,
                "so_sndbuf": 32768,
                "so_rcvbuf": 174760,
$ ../crit show dump/zdtm/live/static/socket-tcp/6299/3/inetsk.img | grep buf
                "so_sndbuf": 10506240,
                "so_rcvbuf": 4242880,
                "so_sndbuf": 65536,
                "so_rcvbuf": 349520,

With-help-of: Pavel Emelyanov <xemul@parallels.com>
Signed-off-by: Andrew Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>