[Unionfs] crash on rm -rf

[Stephen Hemminger]

The following sequence of commands causes a kernel panic when
run on 2.6.26.3 + unionfs 2.4.  It also crashes on 2.6.25

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rm -rf /tmp/active /tmp/changes /tmp/config
mkdir /tmp/active
mkdir /tmp/changes
mkdir /tmp/config

mkdir -p /tmp/active/system/login/user/test1
mkdir -p /tmp/active/system/login/user/test2/password
echo bar >/tmp/active/system/login/user/test2/password/node.val

mount -t unionfs -o dirs=/tmp/changes=rw:/tmp/active=ro unionfs /tmp/config

rm -fr /tmp/config/system/login/user/test2