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>

arch/x86/crtools.c

@@ -475,6 +475,7 @@ int restore_gpregs(struct rt_sigframe *f, UserX86RegsEntry *r)
 	CPREG2(rip, ip);
 	CPREG2(eflags, flags);
 	CPREG1(cs);
+	CPREG1(ss);
 	CPREG1(gs);
 	CPREG1(fs);
 

arch/x86/include/asm/restorer.h

@@ -53,7 +53,7 @@ struct rt_sigcontext {
 	unsigned short			cs;
 	unsigned short			gs;
 	unsigned short			fs;
-	unsigned short			__pad0;
+	unsigned short			ss;
 	unsigned long			err;
 	unsigned long			trapno;
 	unsigned long			oldmask;