[IA64] fix ptrace debug-register handling bug

I noticed that the PTRACE_POKEUSR code incorrectly clears bits 56-58
of _all_ debug registers.  The intention was to only clear it for
odd-numbered registers, to ensure that user-level can only set
user-level data/instruction-breakpoints.  Patch below fixes this problem.
The patch also replaces explicit clearing of the single-step and
taken-branch PSR bits with a call to ptrace_disable() for PTRACE_KILL.
Signed-off-by: David Mosberger-Tang <davidm@hpl.hp.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>

arch/ia64/kernel/ptrace.c

@@ -1052,12 +1052,11 @@ access_uarea (struct task_struct *child, unsigned long addr,
 
 		ptr += regnum;
 
-		if (write_access)
+		if ((regnum & 1) && write_access) {
 			/* don't let the user set kernel-level breakpoints: */
 			*ptr = *data & ~(7UL << 56);
-		else
-			*data = *ptr;
-		return 0;
+			return 0;
+		}
 	}
 	if (write_access)
 		*ptr = *data;
@@ -1521,13 +1520,7 @@ sys_ptrace (long request, pid_t pid, unsigned long addr, unsigned long data)
 			goto out_tsk;
 		child->exit_code = SIGKILL;
 
-		/*
-		 * Make sure the single step/take-branch trap bits are
-		 * not set:
-		 */
-		ia64_psr(pt)->ss = 0;
-		ia64_psr(pt)->tb = 0;
-
+		ptrace_disable(child);
 		wake_up_process(child);
 		ret = 0;
 		goto out_tsk;