illumos gate

found by zfs test:

Test: /opt/zfs-tests/tests/functional/mdb/mdb_001_pos (run as root) [00:15] [FAIL]
00:18:09.57 ASSERTION: Verify that the ZFS mdb dcmds and walkers are working as expected.
00:18:09.57 NOTE: Verifying: '::walk spa'
00:18:10.45 NOTE: Verifying: '::walk spa | ::spa '
00:18:11.21 NOTE: Verifying: '::walk spa | ::spa -c'
00:18:11.85 NOTE: Verifying: '::walk spa | ::spa -v'
00:18:12.47 NOTE: Verifying: '::walk spa | ::spa_config'
00:18:13.06 NOTE: Verifying: '::walk spa | ::spa_space'
00:18:13.90 NOTE: Verifying: '::walk spa | ::spa_space -b'
00:18:14.77 NOTE: Verifying: '::walk spa | ::spa_vdevs'
00:18:15.44 NOTE: Verifying: '::walk spa | ::walk metaslab'
00:18:16.03 NOTE: Verifying: '::walk spa | ::print struct spa spa_root_vdev | ::vdev'
00:18:16.66 NOTE: Verifying: '::walk spa | ::print struct spa spa_root_vdev | ::vdev -re'
00:18:17.28 NOTE: Verifying: '::dbufs'
00:18:18.81 NOTE: Verifying: '::dbufs -n mos -o mdn -l 0 -b 0'
00:18:19.87 NOTE: Verifying: '::dbufs | ::dbuf'
00:18:22.57 NOTE: Verifying: '::dbuf_stats'
00:18:24.68 NOTE: Verifying: '::abuf_find 1 2'
00:18:25.31 mdb '::abuf_find 1 2' contained 'mdb:'
00:18:25.32 mdb: failed to read list element at 0xfffffffffffffffc: no mapping for address
00:18:25.32 mdb: failed to read list element at 0xfffffffffffffffc: no mapping for address
00:18:25.32 mdb: failed to read list element at 0xfffffffffffffffc: no mapping for address
00:18:25.32 mdb: failed to read list element at 0xfffffffffffffffc: no mapping for address
00:18:25.32 NOTE: Performing local cleanup via log_onexit (cleanup)
00:18:25.32 mdb walker or dcmd failed