ARC: pt_regs update #3: Remove unused gutter at start of callee_regs

This is trickier than prev two:

* context switching code saves kernel mode callee regs in the format of
  struct callee_regs thus needs adjustment. This also reduces the height
  of topmost kernel stack frame by 1 word.

* Since kernel stack unwinder is sensitive to height of topmost kernel
  stack frame, that needs a word of adjustment too.

ptrace needs a bit of updating since pt_regs now diverges from
user_regs_struct.

Signed-off-by: Vineet Gupta <vgupta@synopsys.com>

arch/arc/kernel/ptrace.c

@@ -48,6 +48,7 @@ static int genregs_get(struct task_struct *target,
 
 	REG_O_ZERO(pad);
 	REG_O_CHUNK(scratch, callee, ptregs);
+	REG_O_ZERO(pad2);
 	REG_O_CHUNK(callee, efa, cregs);
 	REG_O_CHUNK(efa, stop_pc, &target->thread.fault_address);
 
@@ -96,8 +97,9 @@ static int genregs_set(struct task_struct *target,
 			offsetof(struct user_regs_struct, LOC) + 4);
 
 	REG_IGNORE_ONE(pad);
-	/* TBD: disallow updates to STATUS32, orig_r8 etc*/
-	REG_IN_CHUNK(scratch, callee, ptregs);	/* pt_regs[bta..orig_r8] */
+	/* TBD: disallow updates to STATUS32 etc*/
+	REG_IN_CHUNK(scratch, pad2, ptregs);	/* pt_regs[bta..sp] */
+	REG_IGNORE_ONE(pad2);
 	REG_IN_CHUNK(callee, efa, cregs);	/* callee_regs[r25..r13] */
 	REG_IGNORE_ONE(efa);			/* efa update invalid */
 	REG_IN_ONE(stop_pc, &ptregs->ret);	/* stop_pc: PC update */