pidfs: when time ns disabled add check for ioctl

syzbot call pidfd_ioctl() with cmd "PIDFD_GET_TIME_NAMESPACE" and disabled
CONFIG_TIME_NS, since time_ns is NULL, it will make NULL ponter deref in
open_namespace.

Fixes: 5b08bd40 ("pidfs: allow retrieval of namespace file descriptors") # mainline only
Reported-and-tested-by:  <syzbot+34a0ee986f61f15da35d@syzkaller.appspotmail.com>
Closes: https://syzkaller.appspot.com/bug?extid=34a0ee986f61f15da35d


Signed-off-by: Edward Adam Davis <eadavis@qq.com>
Link: https://lore.kernel.org/r/tencent_7FAE8DB725EE0DD69236DDABDDDE195E4F07@qq.com


Signed-off-by: Christian Brauner <brauner@kernel.org>