restore/x86: restore_gpregs() needs to initialize ->ss as well

Before the recent "x86_64,signal: Fix SS handling for signals delivered
to 64-bit programs" kernel patch, sigreturn paths forgot to restore ->ss
after return from the signal handler.

Now that the kernel was fixed, restore_gpregs() has to initialize ->ss
too, it is no longer ignored.

Note: this is the minimal fix. In the long term we probably should not
dump/restore the segment registers at all. We can use sigcontext filled
by the target kernel and modify the general-purpose regs.

Reported-and-tested-by: Andrey Wagin <avagin@gmail.com>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Cyrill Gorcunov <gorcunov@openvz.org>
Acked-by: Andrew Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>