proc: protect ptrace_may_access() with exec_update_lock (part 1)

In the Linux kernel, the following vulnerability has been resolved: proc: protect ptrace_may_access() with exec_update_lock (part 1) Fix the easy cases where procfs currently calls ptrace_may_access() without exec_update_lock protection, where the fix is to simply add the extra lock or use mm_access(): - do_task_stat(): grab exec_update_lock - proc_pid_wchan(): grab exec_update_lock - proc_map_files_lookup(): use mm_access() instead of get_task_mm() - proc_map_files_readdir(): use mm_access() instead of get_task_mm() - proc_ns_get_link(): grab exec_update_lock - proc_ns_readlink(): grab exec_update_lock