x86, signals: fix xine & firefox bustage

Impact: fix bad frame in rt_sigreturn on 64-bit

After commit 97286a2b some applications
fail to return from signal handler:

[  145.150133] firefox[3250] bad frame in rt_sigreturn frame:00007f902b44eb28 ip:352e80b307 sp:7f902b44ef70 orax:ffffffffffffffff in libpthread-2.9.so[352e800000+17000]
[  665.519017] firefox[5420] bad frame in rt_sigreturn frame:00007faa8deaeb28 ip:352e80b307 sp:7faa8deaef70 orax:ffffffffffffffff in libpthread-2.9.so[352e800000+17000]

The root cause is forgetting to keep 64 byte aligned value of
fpstate for next stack pointer calculation.
Reported-by: Jaswinder Singh Rajput <jaswinder@kernel.org>
Reported-by: Mike Galbraith <efault@gmx.de>
Signed-off-by: Hiroshi Shimamoto <h-shimamoto@ct.jp.nec.com>
LKML-Reference: <49AC85C1.7060600@ct.jp.nec.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

arch/x86/kernel/signal.c

@@ -240,11 +240,10 @@ get_sigframe(struct k_sigaction *ka, struct pt_regs *regs, size_t frame_size,
 
 	if (used_math()) {
 		sp -= sig_xstate_size;
-#ifdef CONFIG_X86_32
-		*fpstate = (void __user *) sp;
-#else /* !CONFIG_X86_32 */
-		*fpstate = (void __user *)round_down(sp, 64);
-#endif /* CONFIG_X86_32 */
+#ifdef CONFIG_X86_64
+		sp = round_down(sp, 64);
+#endif /* CONFIG_X86_64 */
+		*fpstate = (void __user *)sp;
 
 		if (save_i387_xstate(*fpstate) < 0)
 			return (void __user *)-1L;