Illumos openzfs#3955

3955 ztest failure: assertion refcount_count(&tx->tx_space_written) +
     delta <= tx->tx_space_towrite
Reviewed by: Adam Leventhal <ahl@delphix.com>
Reviewed by: Dan Kimmel <dan.kimmel@delphix.com>
Reviewed by: George Wilson <george.wilson@delphix.com>
Approved by: Richard Lowe <richlowe@richlowe.net>

References:
  https://www.illumos.org/issues/3955
  illumos/illumos-gate@be9000c

Ported-by: Richard Yao <ryao@gentoo.org>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue openzfs#1775

cmd/ztest/ztest.c

@@ -3715,6 +3715,9 @@ ztest_dmu_read_write(ztest_ds_t *zd, uint64_t id)
 	else
 		dmu_tx_hold_write(tx, bigobj, bigoff, bigsize);
 
+	/* This accounts for setting the checksum/compression. */
+	dmu_tx_hold_bonus(tx, bigobj);
+
 	txg = ztest_tx_assign(tx, TXG_MIGHTWAIT, FTAG);
 	if (txg == 0) {
 		umem_free(packbuf, packsize);

module/zfs/dmu_tx.c

@@ -465,12 +465,12 @@ dmu_tx_count_free(dmu_tx_hold_t *txh, uint64_t off, uint64_t len)
 		blkid = off >> dn->dn_datablkshift;
 		nblks = (len + dn->dn_datablksz - 1) >> dn->dn_datablkshift;
 
-		if (blkid >= dn->dn_maxblkid) {
+		if (blkid > dn->dn_maxblkid) {
 			rw_exit(&dn->dn_struct_rwlock);
 			return;
 		}
 		if (blkid + nblks > dn->dn_maxblkid)
-			nblks = dn->dn_maxblkid - blkid;
+			nblks = dn->dn_maxblkid - blkid + 1;
 
 	}
 	l0span = nblks;    /* save for later use to calc level > 1 overhead */