backport freeobjects receive handling from 0.7

this combines changes from

e6d3a84

    OpenZFS 6393 - zfs receive a full send as a clone

and

50c957f

    Implement large_dnode pool feature

to hopefully allow sending regular streams from 0.7.x to 0.6.5.x based
systems.the problematic records of the following kind now no longer lead
to an infinite loop, but instead allow the receive to complete:

drr_type = FREEOBJECTS firstobj = 64 numobjs = 36028797018963904 err = 0

see issues openzfs#5699 (older incompatibility between FreeNAS and <= 0.6.5.11)
and openzfs#6507 (recent incompatibility between 0.7.x and <= 0.6.5.11)

module/zfs/dmu_send.c

@@ -1600,22 +1600,30 @@ restore_freeobjects(struct restorearg *ra, objset_t *os,
     struct drr_freeobjects *drrfo)
 {
 	uint64_t obj;
+	int next_err = 0;
 
 	if (drrfo->drr_firstobj + drrfo->drr_numobjs < drrfo->drr_firstobj)
 		return (SET_ERROR(EINVAL));
 
-	for (obj = drrfo->drr_firstobj;
-	    obj < drrfo->drr_firstobj + drrfo->drr_numobjs;
-	    (void) dmu_object_next(os, &obj, FALSE, 0)) {
+	for (obj = drrfo->drr_firstobj == 0 ? 1 : drrfo->drr_firstobj;
+	    obj < drrfo->drr_firstobj + drrfo->drr_numobjs && next_err == 0;
+	    next_err = dmu_object_next(os, &obj, FALSE, 0)) {
 		int err;
 
-		if (dmu_object_info(os, obj, NULL) != 0)
+		err = dmu_object_info(os, obj, NULL);
+		if (err == ENOENT) {
+			obj++;
 			continue;
+		} else if (err != 0) {
+			return (err);
+		}
 
 		err = dmu_free_long_object(os, obj);
 		if (err != 0)
 			return (err);
 	}
+	if (next_err != ESRCH)
+		return (next_err);
 	return (0);
 }