x86: sigframe -- Don't forget to copy ss

Make sure we don't pass any trash value here, because
the kernel does copy it explicitly. We allocate the
memory for frame as zero filled but stack segment
is special and zero is not acceptable (we've had
a discussion on LKML if we need a special handling
for zero ss but end up that new kernels need new CRIU
version, upon which all agreed). Finally in
commit 296bbf7e I managed to hit exactly
this problem :)

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