Fix zdb -c traverse stop on damaged objset root

If a corruption happens to be on a root block of an objset, zdb -c will
not correctly report the error, and it will not traverse the datasets
that come after. This is because traverse_visitbp, which does the
callback and reset error for TRAVERSE_HARD, is skipped when traversing
zil is failed in traverse_impl.

Here's example of what 'zdb -eLcc' command looks like on a pool with
damaged objset root:

== before patch:

Traversing all blocks to verify checksums ...

Error counts:

	errno  count
block traversal size 379392 != alloc 33987072 (unreachable 33607680)

	bp count:             172
	ganged count:           0
	bp logical:       1678336      avg:   9757
	bp physical:       130560      avg:    759     compression:  12.85
	bp allocated:      379392      avg:   2205     compression:   4.42
	bp deduped:             0    ref>1:      0   deduplication:   1.00
	SPA allocated:   33987072     used:  0.80%

	additional, non-pointer bps of type 0:         71
	Dittoed blocks on same vdev: 101

== after patch:

Traversing all blocks to verify checksums ...

zdb_blkptr_cb: Got error 52 reading <54, 0, -1, 0>  -- skipping

Error counts:

	errno  count
	   52  1
block traversal size 33963520 != alloc 33987072 (unreachable 23552)

	bp count:             447
	ganged count:           0
	bp logical:      36093440      avg:  80745
	bp physical:     33699840      avg:  75391     compression:   1.07
	bp allocated:    33963520      avg:  75981     compression:   1.06
	bp deduped:             0    ref>1:      0   deduplication:   1.00
	SPA allocated:   33987072     used:  0.80%

	additional, non-pointer bps of type 0:         76
	Dittoed blocks on same vdev: 115

==

Signed-off-by: Chunwei Chen <david.chen@nutanix.com>

module/zfs/dmu_traverse.c

@@ -634,12 +634,20 @@ traverse_impl(spa_t *spa, dsl_dataset_t *ds, uint64_t objset, blkptr_t *rootbp,
 
 		err = arc_read(NULL, td->td_spa, rootbp, arc_getbuf_func,
 		    &buf, ZIO_PRIORITY_ASYNC_READ, zio_flags, &flags, czb);
-		if (err != 0)
-			return (err);
-
-		osp = buf->b_data;
-		traverse_zil(td, &osp->os_zil_header);
-		arc_buf_destroy(buf, &buf);
+		if (err != 0) {
+			/*
+			 * If both TRAVERSE_HARD and TRAVERSE_PRE are set,
+			 * continue to visitbp so that td_func can be called
+			 * in pre stage, and err will reset to zero.
+			 */
+			if (!(td->td_flags & TRAVERSE_HARD) ||
+			    !(td->td_flags & TRAVERSE_PRE))
+				return (err);
+		} else {
+			osp = buf->b_data;
+			traverse_zil(td, &osp->os_zil_header);
+			arc_buf_destroy(buf, &buf);
+		}
 	}
 
 	if (!(flags & TRAVERSE_PREFETCH_DATA) ||