send_iterate_snap : doall send without fromsnap

The behavior of a NULL fromsnap was inadvertently changed for a doall
send when the send/recv logic in libzfs was updated. Restore the
previous behavior by correcting send_iterate_snap() to include all
the snapshots in the nvlist for this case.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Cedric Maunoury <cedric.maunoury@gmail.com>
Closes #11608