OpenZFS 6940 - Cannot unlink directories when over quota

From user perspective, I would expect that ZFS is always able
to remove files and directories even when the quota is exceeded.

Authored by: Simon Klinkert <simon.klinkert@gmail.com>
Reviewed by: Dan McDonald <danmcd@omniti.com>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Approved by: Robert Mustacchi <rm@joyent.com>
Ported-by: kernelOfTruth kerneloftruth@gmail.com
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>

OpenZFS-issue: https://www.illumos.org/issues/6940
OpenZFS-issue: https://www.illumos.org/issues/6334
OpenZFS-commit: illumos/illumos-gate@9918916
Closes #5044

Author: sklinkert

module/zfs/zfs_vnops.c

@@ -2003,6 +2003,7 @@ zfs_rmdir(struct inode *dip, char *name, struct inode *cwd, cred_t *cr,
 	dmu_tx_hold_zap(tx, zsb->z_unlinkedobj, FALSE, NULL);
 	zfs_sa_upgrade_txholds(tx, zp);
 	zfs_sa_upgrade_txholds(tx, dzp);
+	dmu_tx_mark_netfree(tx);
 	error = dmu_tx_assign(tx, waited ? TXG_WAITED : TXG_NOWAIT);
 	if (error) {
 		rw_exit(&zp->z_parent_lock);