Fix i/o error handling of livelists and zap iteration

Pool-wide metadata is stored in the MOS (Meta Object Set).  This
metadata is stored in triplicate, in addition to any pool-level
reduncancy (e.g. RAIDZ).  However, if all 3+ copies of this metadata are
not available, we can still get EIO/ECKSUM when reading from the MOS.
If we encounter such an error in syncing context, we have typically
already committed to making a change that we now can't do because of the
corrupt/missing metadata.  We typically "handle" this with a `VERIFY()`
or `zfs_panic_recover()`.  This prevents the system from continuing on
in an undefined state, while minimizing the amount of error-handling
code.

However, there are some code paths that ignore these i/o errors, or
`ASSERT()` that they don't happen.  Since assertions are disabled on
non-debug builds, they effectively ignore them as well.  This can lead
to ZFS continuing on in an incorrect state, potentially leading to
on-disk inconsistencies.

This commit adds handling for these i/o errors on MOS metadata,
typically with a `VERIFY()`:

* Handle error return from `zap_cursor_retrieve()` in 4 places in
`dsl_deadlist.c`.

* Handle error return from `zap_contains()` in `dsl_dir_hold_obj()`.
Turns out this call isn't necessary because we can always call
`zap_lookup()`.

* Handle error return from `zap_lookup()` in `dsl_fs_ss_limit_check()`.

* Handle error return from `zap_remove()` in `dsl_dir_rename_sync()`.

* Handle error return from `zap_lookup()` in
`dsl_dir_remove_livelist()`.

* Handle error return from `dsl_process_sub_livelist()` in
`spa_livelist_delete_cb()`.

Additionally:

* Augment the internal history log message for `zfs destroy` to note
which method is used (e.g. bptree, livelist, or, synchronous) and the
mintxg.

* Correct a comment in `dbuf_init()`.

* Correct indentation in `dsl_dir_remove_livelist()`.

Reviewed by: Sara Hartse <sara.hartse@delphix.com>
Reviewed-by: George Wilson <george.wilson@delphix.com>
Signed-off-by: Matthew Ahrens <mahrens@delphix.com>
Closes #10643

module/zfs/spa.c

@@ -2527,7 +2527,7 @@ spa_livelist_delete_cb(void *arg, zthr_t *z)
 			    sublist_delete_sync, &sync_arg, 0,
 			    ZFS_SPACE_CHECK_DESTROY));
 		} else {
-			ASSERT(err == EINTR);
+			VERIFY3U(err, ==, EINTR);
 		}
 		bplist_clear(&to_free);
 		bplist_destroy(&to_free);