pidfd: remove unneeded NULL check from pidfd_prepare()

None of the caller actually pass a NULL pid in there.

Link: https://lore.kernel.org/r/[e-mail]
Reviewed-by: Oleg Nesterov <[e-mail]>
Signed-off-by: Christian Brauner <[e-mail]>

kernel/fork.c

···
 {
 	bool thread = flags & PIDFD_THREAD;
 
-	if (!pid || !pid_has_task(pid, thread ? PIDTYPE_PID : PIDTYPE_TGID))
+	if (!pid_has_task(pid, thread ? PIDTYPE_PID : PIDTYPE_TGID))
 		return -EINVAL;
 
 	return __pidfd_prepare(pid, flags, ret);