Allow sending corrupt snapshots even if metadata is corrupted

When zfs_send_corrupt_data is set, use the TRAVERSE_HARD flag,
so traverse_visitbp() will not fail with ECKSUM if a blockpointer
cannot be read, but rather will continue and send the objects it can.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: John Kennedy <john.kennedy@delphix.com>
Signed-off-by: Allan Jude <allan@klarasystems.com>
Sponsored-By: Klara Inc.
Sponsored-By: WHC Online Solutions Inc.
Closes openzfs#12541

module/zfs/dmu_send.c

@@ -2054,6 +2054,8 @@ setup_to_thread(struct send_thread_arg *to_arg, objset_t *to_os,
 	to_arg->flags = TRAVERSE_PRE | TRAVERSE_PREFETCH_METADATA;
 	if (rawok)
 		to_arg->flags |= TRAVERSE_NO_DECRYPT;
+	if (zfs_send_corrupt_data)
+		to_arg->flags |= TRAVERSE_HARD;
 	to_arg->num_blocks_visited = &dssp->dss_blocks;
 	(void) thread_create(NULL, 0, send_traverse_thread, to_arg, 0,
 	    curproc, TS_RUN, minclsyspri);