Add a pool containing unprocessed deferred destroy operations.

[dweeezil]

The pool contains the "bptree_obj" entry in the MOS object directory.
It's DMU object contains a pair of bptree_entry_phys_t items, one for
each of a pair of destroyed small filesystems. Although ZoL 0.6.2 was
used to create the pool, it was modified to not process the deferred
destroy list in order to cleanly produce a pool in this state.

[dweeezil]

When this pool is imported into a version of ZoL with the openzfs/zfs#2094 erratum, the following ASSERT is hit when importing the pool due to the odd-sized zbookmark_t:

[ 6505.839469] SPLError: 19585:0:(dsl_dir.c:858:dsl_dir_diduse_space()) ASSERTION(used >= 0 || dd->dd_phys->dd_used_bytes >= -used) failed
[ 6505.841228] SPLError: 19585:0:(dsl_dir.c:858:dsl_dir_diduse_space()) SPL PANIC
[ 6505.841245] SPLError: 19576:0:(avl.c:222:avl_nearest()) ASSERTION(tree->avl_root == NULL) failed
[ 6505.841246] SPLError: 19576:0:(avl.c:222:avl_nearest()) SPL PANIC

On a version of ZoL with the erratum, several problems are observed with zdb (on the exported pool):

• For a plain zdb, an assert is hit unless zdb -L is used
• Not only will a zdb -ddddd will not show all the blocks that should be freed, but at least one of them displays a bad checksum (verified by diffing zdb output).

[behlendorf]

@dweeezil Thanks for constructing this pool. I haven't had a chance to investigate the ASSERTs yet but I plan too.