tracehook: fix CLONE_PTRACE

In the change in commit 09a05394fe2448a4139b014936330af23fa7ec83, I
overlooked two nits in the logic and this broke using CLONE_PTRACE
when PTRACE_O_TRACE* are not being used.

A parent that is itself traced at all but not using PTRACE_O_TRACE*,
using CLONE_PTRACE would have its new child fail to be traced.

A parent that is not itself traced at all that uses CLONE_PTRACE
(which should be a no-op in this case) would confuse the bookkeeping
and lead to a crash at exit time.

This restores the missing checks and fixes both failure modes.

Reported-by: Eduardo Habkost <[e-mail]>
Signed-off-by: Roland McGrath <[e-mail]>

include/linux/ptrace.h

···
 	child->ptrace = 0;
 	if (unlikely(ptrace)) {
 		child->ptrace = current->ptrace;
-		__ptrace_link(child, current->parent);
+		ptrace_link(child, current->parent);
 	}
 }
 

include/linux/tracehook.h

···
 					  unsigned long clone_flags,
 					  pid_t pid, struct task_struct *child)
 {
-	if (unlikely(trace)) {
+	if (unlikely(trace) || unlikely(clone_flags & CLONE_PTRACE)) {
 		/*
 		 * The child starts up with an immediate SIGSTOP.
 		 */