avr32: Fix sigaltstack behaviour

A signal handler should be able to change the signal stack used for the
next signal by altering the ucontext_t passed as a parameter to the
handler. This does not currently work on avr32 since it doesn't update
the in-kernel signal context from the ucontext_t upon signal handler
return.

Fix it by adding a call to do_sigaltstack() from sys_rt_sigreturn(),
bringing it in line with most other architectures.
Signed-off-by: Martin Koegler <mkoegler@auto.tuwien.ac.at>
[haavard.skinnemoen@atmel.com: changed patch description]
Signed-off-by: Haavard Skinnemoen <haavard.skinnemoen@atmel.com>

arch/avr32/kernel/signal.c

@@ -93,6 +93,9 @@ asmlinkage int sys_rt_sigreturn(struct pt_regs *regs)
 	if (restore_sigcontext(regs, &frame->uc.uc_mcontext))
 		goto badframe;
 
+	if (do_sigaltstack(&frame->uc.uc_stack, NULL, regs->sp) == -EFAULT)
+		goto badframe;
+
 	pr_debug("Context restored: pc = %08lx, lr = %08lx, sp = %08lx\n",
 		 regs->pc, regs->lr, regs->sp);