ARC: SA_SIGINFO ucontext regs off-by-one

The regfile provided to SA_SIGINFO signal handler as ucontext was off by
one due to pt_regs gutter cleanups in 2013.

Before handling signal, user pt_regs are copied onto user_regs_struct and copied
back later. Both structs are binary compatible. This was all fine until
commit 2fa919045b (ARC: pt_regs update #2) which removed the empty stack slot
at top of pt_regs (corresponding to first pad) and made the corresponding
fixup in struct user_regs_struct (the pad in there was moved out of
@scratch - not removed altogether as it is part of ptrace ABI)

 struct user_regs_struct {
+       long pad;
        struct {
-               long pad;
                long bta, lp_start, lp_end,....
        } scratch;
 ...
 }

This meant that now user_regs_struct was off by 1 reg w.r.t pt_regs and
signal code needs to user_regs_struct.scratch to reflect it as pt_regs,
which is what this commit does.

This problem was hidden for 2 years, because both save/restore, despite
using wrong location, were using the same location. Only an interim
inspection (reproducer below) exposed the issue.

     void handle_segv(int signo, siginfo_t *info, void *context)
     {
 	ucontext_t *uc = context;
	struct user_regs_struct *regs = &(uc->uc_mcontext.regs);

	printf("regs %x %x\n",               <=== prints 7 8 (vs. 8 9)
               regs->scratch.r8, regs->scratch.r9);
     }

     int main()
     {
	struct sigaction sa;

	sa.sa_sigaction = handle_segv;
	sa.sa_flags = SA_SIGINFO;
	sigemptyset(&sa.sa_mask);
	sigaction(SIGSEGV, &sa, NULL);

	asm volatile(
	"mov	r7, 7	\n"
	"mov	r8, 8	\n"
	"mov	r9, 9	\n"
	"mov	r10, 10	\n"
	:::"r7","r8","r9","r10");

	*((unsigned int*)0x10) = 0;
     }

Fixes: 2fa919045b "ARC: pt_regs update #2: Remove unused gutter at start of pt_regs"
CC: <stable@vger.kernel.org>
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>

arch/arc/kernel/signal.c

@@ -67,7 +67,7 @@ stash_usr_regs(struct rt_sigframe __user *sf, struct pt_regs *regs,
 	       sigset_t *set)
 {
 	int err;
-	err = __copy_to_user(&(sf->uc.uc_mcontext.regs), regs,
+	err = __copy_to_user(&(sf->uc.uc_mcontext.regs.scratch), regs,
 			     sizeof(sf->uc.uc_mcontext.regs.scratch));
 	err |= __copy_to_user(&sf->uc.uc_sigmask, set, sizeof(sigset_t));
 
@@ -83,7 +83,7 @@ static int restore_usr_regs(struct pt_regs *regs, struct rt_sigframe __user *sf)
 	if (!err)
 		set_current_blocked(&set);
 
-	err |= __copy_from_user(regs, &(sf->uc.uc_mcontext.regs),
+	err |= __copy_from_user(regs, &(sf->uc.uc_mcontext.regs.scratch),
 				sizeof(sf->uc.uc_mcontext.regs.scratch));
 
 	return err;