sh64: fix altstack switching on sigreturn

incidentally, declaring a local variable as __user (!) to make
sparse STFU is really sick.  Especially since sparse had been
100% right - it *is* a bug.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

arch/sh/kernel/signal_64.c

@@ -347,7 +347,6 @@ asmlinkage int sys_rt_sigreturn(unsigned long r2, unsigned long r3,
 {
 	struct rt_sigframe __user *frame = (struct rt_sigframe __user *) (long) REF_REG_SP;
 	sigset_t set;
-	stack_t __user st;
 	long long ret;
 
 	/* Always make any pending restarted system calls return -EINTR */
@@ -365,11 +364,10 @@ asmlinkage int sys_rt_sigreturn(unsigned long r2, unsigned long r3,
 		goto badframe;
 	regs->pc -= 4;
 
-	if (__copy_from_user(&st, &frame->uc.uc_stack, sizeof(st)))
-		goto badframe;
 	/* It is more difficult to avoid calling this function than to
 	   call it and ignore errors.  */
-	do_sigaltstack(&st, NULL, REF_REG_SP);
+	if (do_sigaltstack(&frame->uc.uc_stack, NULL, REF_REG_SP) == -EFAULT)
+		goto badframe;
 
 	return (int) ret;