relabel check for MLS rather than returning 0 directly.
This problem didn't result in a vulnerability currently as the central
implementation of ifnet relabeling also checks for UNIX privilege, and
we currently don't guarantee containment for the root user in mac_mls,
but we should be using the MLS definition of privilege as well as the
UNIX definition in anticipation of supporting root containment at some
point.
MFC after: 3 days
Submitted by: Zhouyi Zhou <zhouzhouyi at gmail dot com>
Sponsored by: Google SoC 2007
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