overlay: create directory over deleted whiteout

overlay: create directory over deleted whiteout

There's a bug in the overlayfs implementation starting from the very
first merged version that may cause an Oops of various forms if a
directory is created over a whiteout dentry, but the actual whiteout
on the upper layer was removed to the directory creation.

Reported by: kaixuxia <xiakaixu1987@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>