x86_32, signal: Fix vdso rt_sigreturn

This commit:

    commit 6f121e54
    Author: Andy Lutomirski <luto@amacapital.net>
    Date:   Mon May 5 12:19:34 2014 -0700

        x86, vdso: Reimplement vdso.so preparation in build-time C

Contained this obvious typo:

-               restorer = VDSO32_SYMBOL(current->mm->context.vdso, rt_sigreturn);
+               restorer = current->mm->context.vdso +
+                       selected_vdso32->sym___kernel_sigreturn;

Note the missing 'rt_' in the new code.  Fix it.
Signed-off-by: Andy Lutomirski <luto@amacapital.net>
Link: http://lkml.kernel.org/r/1eb40ad923acde2e18357ef2832867432e70ac42.1403361010.git.luto@amacapital.netSigned-off-by: H. Peter Anvin <hpa@linux.intel.com>

arch/x86/kernel/signal.c

@@ -363,7 +363,7 @@ static int __setup_rt_frame(int sig, struct ksignal *ksig,
 
 		/* Set up to return from userspace.  */
 		restorer = current->mm->context.vdso +
-			selected_vdso32->sym___kernel_sigreturn;
+			selected_vdso32->sym___kernel_rt_sigreturn;
 		if (ksig->ka.sa.sa_flags & SA_RESTORER)
 			restorer = ksig->ka.sa.sa_restorer;
 		put_user_ex(restorer, &frame->pretcode);