Fix 'zfs list -t snapshot' depth

Fix 'zfs list -t snapshot' depth

Commit df583073 introduced the ability to list the snapshots for a
specified dataset.  This change inadvertently resulted in only the top-
level snapshots being listed when no dataset was specified.  Fix this
issue by adding an additional check to determine if a dataset was
provided to avoid incorrectly restricting the depth.

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Tom Caputi <tcaputi@datto.com>
Reviewed-by: Alek Pinchuk <apinchuk@datto.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #8591
Closes #8594