powerpc: Fix booke user_disable_single_step()

On booke processors, gdb is seeing spurious SIGTRAPs when setting a
watchpoint.

user_disable_single_step() simply quits when the DAC is non-zero.  It should
be clearing the DBCR0_IC and DBCR0_BT bits from the dbcr0 register and
TIF_SINGLESTEP from the thread flag.
Signed-off-by: Dave Kleikamp <shaggy@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/kernel/ptrace.c

@@ -736,15 +736,16 @@ void user_disable_single_step(struct task_struct *task)
 {
 	struct pt_regs *regs = task->thread.regs;
 
-
+	if (regs != NULL) {
 #if defined(CONFIG_BOOKE)
-	/* If DAC then do not single step, skip */
+		/* If DAC don't clear DBCRO_IDM or MSR_DE */
 		if (task->thread.dabr)
-		return;
-#endif
-
-	if (regs != NULL) {
-#if defined(CONFIG_40x) || defined(CONFIG_BOOKE)
+			task->thread.dbcr0 &= ~(DBCR0_IC | DBCR0_BT);
+		else {
+			task->thread.dbcr0 &= ~(DBCR0_IC | DBCR0_BT | DBCR0_IDM);
+			regs->msr &= ~MSR_DE;
+		}
+#elif defined(CONFIG_40x)
 		task->thread.dbcr0 &= ~(DBCR0_IC | DBCR0_BT | DBCR0_IDM);
 		regs->msr &= ~MSR_DE;
 #else