ptrace/powerpc: Don't flush_ptrace_hw_breakpoint() on fork()

arch_dup_task_struct() does flush_ptrace_hw_breakpoint(src), this
destroys the parent's breakpoints for no reason. We should clear
child->thread.ptrace_bps[] copied by dup_task_struct() instead.
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Michael Neuling <mikey@neuling.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/kernel/process.c

@@ -910,10 +910,6 @@ int arch_dup_task_struct(struct task_struct *dst, struct task_struct *src)
 	flush_altivec_to_thread(src);
 	flush_vsx_to_thread(src);
 	flush_spe_to_thread(src);
-#ifdef CONFIG_HAVE_HW_BREAKPOINT
-	flush_ptrace_hw_breakpoint(src);
-#endif /* CONFIG_HAVE_HW_BREAKPOINT */
-
 	*dst = *src;
 	return 0;
 }
@@ -984,6 +980,10 @@ int copy_thread(unsigned long clone_flags, unsigned long usp,
 	p->thread.ksp_limit = (unsigned long)task_stack_page(p) +
 				_ALIGN_UP(sizeof(struct thread_info), 16);
 
+#ifdef CONFIG_HAVE_HW_BREAKPOINT
+	p->thread.ptrace_bps[0] = NULL;
+#endif
+
 #ifdef CONFIG_PPC_STD_MMU_64
 	if (mmu_has_feature(MMU_FTR_SLB)) {
 		unsigned long sp_vsid;