Disable system call restart at sigreturn time rather than when

invoking the signal. This fixes all races.

As per discussion with Paul Mackerras on linux-kernel.

arch/i386/kernel/signal.c

@@ -132,6 +132,9 @@ restore_sigcontext(struct pt_regs *regs, struct sigcontext __user *sc, int *peax
 {
 	unsigned int err = 0;
 
+	/* Always make any pending restarted system calls return -EINTR */
+	current_thread_info()->restart_block.fn = do_no_restart_syscall;
+
 #define COPY(x)		err |= __get_user(regs->x, &sc->x)
 
 #define COPY_SEG(seg)							\
@@ -504,9 +507,6 @@ handle_signal(unsigned long sig, siginfo_t *info, sigset_t *oldset,
 {
 	struct k_sigaction *ka = &current->sighand->action[sig-1];
 
-	/* Always make any pending restarted system calls return -EINTR */
-	current_thread_info()->restart_block.fn = do_no_restart_syscall;
-
 	/* Are we from a system call? */
 	if (regs->orig_eax >= 0) {
 		/* If so, check system call restarting.. */