xfstests: mount with security context if SELinux is enabled

Index: xfs-cmds/xfstests/common.rc
===================================================================
@@ -39,9 +39,18 @@ dd()
 
 _mount_opts()
 {
+    # SELinux adds extra xattrs which can mess up our expected output.
+    # So, mount with a context, and they won't be created
+    # nfs_t is a "liberal" context so we can use it.
+    if [ "$HOSTOS" == "Linux" ]; then
+	if [ -x /usr/sbin/selinuxenabled ] && /usr/sbin/selinuxenabled; then
+		SELINUX_OPTIONS="-o context=system_u:object_r:nfs_t:s0"
+	fi
+    fi
+
     case $FSTYP in
     xfs)
-	export MOUNT_OPTIONS=$XFS_MOUNT_OPTIONS
+	export MOUNT_OPTIONS="$XFS_MOUNT_OPTIONS $SELINUX_OPTIONS"
 	;;
     udf)
 	export MOUNT_OPTIONS=$UDF_MOUNT_OPTIONS