pidns: kill the unnecessary CLONE_NEWPID in copy_process()

Commit 8382fcac1b81 ("pidns: Outlaw thread creation after
unshare(CLONE_NEWPID)") nacks CLONE_NEWPID if the forking process
unshared pid_ns.  This is correct but unnecessary, copy_pid_ns() does
the same check.

Remove the CLONE_NEWPID check to cleanup the code and prepare for the
next change.

Test-case:

	static int child(void *arg)
	{
		return 0;
	}

	static char stack[16 * 1024];

	int main(void)
	{
		pid_t pid;

		assert(unshare(CLONE_NEWUSER | CLONE_NEWPID) == 0);

		pid = clone(child, stack + sizeof(stack) / 2,
				CLONE_NEWPID | SIGCHLD, NULL);
		assert(pid < 0 && errno == EINVAL);

		return 0;
	}

clone(CLONE_NEWPID) correctly fails with or without this change.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Andy Lutomirski <luto@amacapital.net>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Colin Walters <walters@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/fork.c

@@ -1177,9 +1177,8 @@ static struct task_struct *copy_process(unsigned long clone_flags,
 	 * allow it to share a thread group or signal handlers with the
 	 * forking task.
 	 */
-	if ((clone_flags & (CLONE_SIGHAND | CLONE_NEWPID)) &&
-	    (task_active_pid_ns(current) !=
-	     current->nsproxy->pid_ns_for_children))
+	if ((clone_flags & CLONE_SIGHAND) && (task_active_pid_ns(current) !=
+					current->nsproxy->pid_ns_for_children))
 		return ERR_PTR(-EINVAL);
 
 	retval = security_task_create(clone_flags);