powerpc/booke: clear DBCR0_BT in user_disable_single_step()

BookE version of user_disable_single_step() clears DBCR0_IC for the
instruction completion debug, but did not also clear DBCR0_BT for the
branch taken exception.  This behavior was lost by the 2/2010 patch.
Signed-off-by: NJames Yang <James.Yang@freescale.com>
Signed-off-by: NScott Wood <scottwood@freescale.com>

arch/powerpc/kernel/ptrace.c

@@ -895,7 +895,7 @@ void user_disable_single_step(struct task_struct *task)
 		 * And, after doing so, if all debug flags are off, turn
 		 * off DBCR0(IDM) and MSR(DE) .... Torez
 		 */
-		task->thread.debug.dbcr0 &= ~DBCR0_IC;
+		task->thread.debug.dbcr0 &= ~(DBCR0_IC|DBCR0_BT);
 		/*
 		 * Test to see if any of the DBCR_ACTIVE_EVENTS bits are set.
 		 */