Fix the build when INVARIANTS is defined by restoring bo's definition in

ext2_truncate(..) and by putting it under INVARIANTS ifdefs

X-MFC with: r277354
MFC after: 2 weeks

sys/fs/ext2fs/ext2_inode.c

@@ -121,8 +121,14 @@ ext2_truncate(struct vnode *vp, off_t length, int flags, struct ucred *cred,
 	e4fs_daddr_t count, nblocks, blocksreleased = 0;
 	int error, i, allerror;
 	off_t osize;
+#ifdef INVARIANTS
+	struct bufobj *bo;
+#endif
 
 	oip = VTOI(ovp);
+#ifdef INVARIANTS
+	bo = &ovp->v_bufobj;
+#endif
 
 	ASSERT_VOP_LOCKED(vp, "ext2_truncate");