powerpc/perf: Ensure all EBB register state is cleared on fork()

In commit 330a1eb "Core EBB support for 64-bit book3s" I messed up
clear_task_ebb(). It clears some but not all of the task's Event Based
Branch (EBB) registers when we duplicate a task struct.

That allows a child task to observe the EBBHR & EBBRR of its parent,
which it should not be able to do.

Fix it by clearing EBBHR & EBBRR.

Signed-off-by: Michael Ellerman <[e-mail]>
Cc: [e-mail] [v3.11+]
Signed-off-by: Benjamin Herrenschmidt <[e-mail]>

arch/powerpc/include/asm/switch_to.h

···
 {
 #ifdef CONFIG_PPC_BOOK3S_64
     /* EBB perf events are not inherited, so clear all EBB state. */
+    t->thread.ebbrr = 0;
+    t->thread.ebbhr = 0;
     t->thread.bescr = 0;
     t->thread.mmcr2 = 0;
     t->thread.mmcr0 = 0;