sparc64: Fix sparc64_set_context stack handling.

Like a signal return, we should use synchronize_user_stack() rather
than flush_user_windows().
Reported-by: NIlya Malakhov <ilmalakhovthefirst@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

arch/sparc/kernel/signal_64.c

@@ -52,7 +52,7 @@ asmlinkage void sparc64_set_context(struct pt_regs *regs)
 	unsigned char fenab;
 	int err;
 
-	flush_user_windows();
+	synchronize_user_stack();
 	if (get_thread_wsaved()					||
 	    (((unsigned long)ucp) & (sizeof(unsigned long)-1))	||
 	    (!__access_ok(ucp, sizeof(*ucp))))