`OverlayConvertWhiteout` calls `mknod c 0 0` which is not allowed when
running in a user namespace, even in Ubuntu kernel.
Although there is an alternative hacky way to create whiteouts without
calling mknod as Moby `overlay2` actually does(see #3762), let's use
naive applier when running in UserNS and call it a day.
Close#3762
Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>
