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

sys/security/mac_mls/mac_mls.c

@@ -964,9 +964,7 @@ mls_ifnet_check_relabel(struct ucred *cred, struct ifnet *ifp,
 	/*
 	 * Relabeling network interfaces requires MLS privilege.
 	 */
-	error = mls_subject_privileged(subj);
-
-	return (0);
+	return (mls_subject_privileged(subj));
 }
 
 static int