Illumos 4950 - files sometimes can't be removed from a full filesystem

4950 files sometimes can't be removed from a full filesystem
Reviewed by: Adam Leventhal <adam.leventhal@delphix.com>
Reviewed by: George Wilson <george.wilson@delphix.com>
Reviewed by: Sebastien Roy <sebastien.roy@delphix.com>
Reviewed by: Boris Protopopov <bprotopopov@hotmail.com>
Approved by: Dan McDonald <danmcd@omniti.com>

References:
  https://www.illumos.org/issues/4950
  https://github.com/illumos/illumos-gate/commit/4bb7380

Porting notes:
- ZoL currently does not log discards to zvols, so the portion of
  this patch that modifies the discard logging to mark it as
  freeing space has been discarded.

2. may_delete_now had been removed from zfs_remove() in ZoL.
   It has been reintroduced.

3. We do not try to emulate vnodes, so the following lines are
   not valid on Linux:

	mutex_enter(&vp->v_lock);
	may_delete_now = vp->v_count == 1 && !vn_has_cached_data(vp);
	mutex_exit(&vp->v_lock);

  This has been replaced with:

	mutex_enter(&zp->z_lock);
	may_delete_now = atomic_read(&ip->i_count) == 1 && !(zp->z_is_mapped);
	mutex_exit(&zp->z_lock);

Ported-by: Richard Yao <richard.yao@clusterhq.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>

module/zfs/dmu.c

@@ -687,6 +687,12 @@ dmu_free_long_range_impl(objset_t *os, dnode_t *dn, uint64_t offset,
 		tx = dmu_tx_create(os);
 		dmu_tx_hold_free(tx, dn->dn_object,
 		    chunk_begin, chunk_end - chunk_begin);
+
+		/*
+		 * Mark this transaction as typically resulting in a net
+		 * reduction in space used.
+		 */
+		dmu_tx_mark_netfree(tx);
 		err = dmu_tx_assign(tx, TXG_WAIT);
 		if (err) {
 			dmu_tx_abort(tx);
@@ -738,6 +744,7 @@ dmu_free_long_object(objset_t *os, uint64_t object)
 	tx = dmu_tx_create(os);
 	dmu_tx_hold_bonus(tx, object);
 	dmu_tx_hold_free(tx, object, 0, DMU_OBJECT_END);
+	dmu_tx_mark_netfree(tx);
 	err = dmu_tx_assign(tx, TXG_WAIT);
 	if (err == 0) {
 		err = dmu_object_free(os, object, tx);