ovl: check whiteout on lowest layer as well

Not checking whiteouts on lowest layer was an optimization (there's nothing
to white out there), but it could result in inconsitent behavior when a
layer previously used as upper/middle is later used as lowest.

Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>

diff --cc fs/overlayfs/readdir.c
Simple merge

diff --cc fs/overlayfs/super.c
Simple merge