Properly return the error from mls_subject_privileged() in the ifnet relabel check for MLS rather than returning 0 directly.
Properly return the error from mls_subject_privileged() in the ifnet
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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