Stack overflow when destroying deeply nested clones

Stack overflow when destroying deeply nested clones

Destroy operations on deeply nested chains of clones can overflow
the stack:

        Depth    Size   Location    (221 entries)
        -----    ----   --------
  0)    15664      48   mutex_lock+0x5/0x30
  1)    15616       8   mutex_lock+0x5/0x30
...
 26)    13576      72   dsl_dataset_remove_clones_key.isra.4+0x124/0x1e0 [zfs]
 27)    13504      72   dsl_dataset_remove_clones_key.isra.4+0x18a/0x1e0 [zfs]
 28)    13432      72   dsl_dataset_remove_clones_key.isra.4+0x18a/0x1e0 [zfs]
...
185)     2128      72   dsl_dataset_remove_clones_key.isra.4+0x18a/0x1e0 [zfs]
186)     2056      72   dsl_dataset_remove_clones_key.isra.4+0x18a/0x1e0 [zfs]
187)     1984      72   dsl_dataset_remove_clones_key.isra.4+0x18a/0x1e0 [zfs]
188)     1912     136   dsl_destroy_snapshot_sync_impl+0x4e0/0x1090 [zfs]
189)     1776      16   dsl_destroy_snapshot_check+0x0/0x90 [zfs]
...
218)      304     128   kthread+0xdf/0x100
219)      176      48   ret_from_fork+0x22/0x40
220)      128     128   kthread+0x0/0x100

Fix this issue by converting dsl_dataset_remove_clones_key() from
recursive to iterative.

Reviewed-by: Paul Zuchowski <pzuchowski@datto.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: loli10K <ezomori.nozomu@gmail.com>
Closes #7279
Closes #7810