sockets: don't double buffer size for sockets

SO_SNDBUF and SO_RCVBUF sets a double input value, because
"Linux reserves half of te socket buffer for metadata."

So if a process is suspended/restored many times, a socket buffer
size is doubled on each iteration and in a one moment it is overflowed.
Very likely the program hangs in such situation, because the socket with
negative buffer size is unusable.

Signed-off-by: Andrey Vagin <avagin@openvz.org>
Acked-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>