MFC r211598:

MFC r211598:

  Add support for whiteouts on tmpfs.

  Right now unionfs only allows filesystems to be mounted on top of
  another if it supports whiteouts. Even though I have sent a patch to
  daichi@ to let unionfs work without it, we'd better also add support for
  whiteouts to tmpfs.

  This patch implements .vop_whiteout and makes necessary changes to
  lookup() and readdir() to take them into account. We must also make sure
  that when adding or removing a file, we honour the componentname's
  DOWHITEOUT and ISWHITEOUT, to prevent duplicate filenames.

git-svn-id: svn://svn.freebsd.org/base/stable/8@212953 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f