Destroy makes full snap list before destroying

Change zfs destroy logic so destroying begins before
the entire list of snapshots is built.

Reviewed-by: loli10K <ezomori.nozomu@gmail.com>
Reviewed-by: Kash Pande <kash@tripleback.net>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Paul Zuchowski <pzuchowski@datto.com>
Closes #7271