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 #12541