Kernel: Only allow tracing stopped threads

Previously, this allowed tracing Runnable threads (or threads in any
other state other than Running).

Allowing threads to be traced while they are runnable doesn't seem
like a good idea, especially since they can be scheduled while the
tracer thread is still in the ptrace() syscall.

Other Unixes seem to only allow tracing stopped threads as well.

Kernel/Syscalls/ptrace.cpp

@@ -68,7 +68,7 @@ static ErrorOr<FlatPtr> handle_ptrace(Kernel::Syscall::SC_ptrace_params const& p
     if (tracer->tracer_pid() != caller.pid())
         return EBUSY;
 
-    if (peer->state() == Thread::State::Running)
+    if (peer->state() != Thread::State::Stopped)
         return EBUSY;
 
     scheduler_lock.unlock();