ovl: check whiteout in ovl_create_over_whiteout()

ovl: check whiteout in ovl_create_over_whiteout()

Kaixuxia repors that it's possible to crash overlayfs by removing the
whiteout on the upper layer before creating a directory over it.  This is a
reproducer:

 mkdir lower upper work merge
 touch lower/file
 mount -t overlay overlay -olowerdir=lower,upperdir=upper,workdir=work merge
 rm merge/file
 ls -al merge/file
 rm upper/file
 ls -al merge/
 mkdir merge/file

Before commencing with a vfs_rename(..., RENAME_EXCHANGE) verify that the
lookup of "upper" is positive and is a whiteout, and return ESTALE
otherwise.

Reported by: kaixuxia <[email protected]>
Signed-off-by: Miklos Szeredi <[email protected]>
Fixes: e9be9d5e76e3 ("overlay filesystem")
Cc: <[email protected]> # v3.18