walk doesn't work on process targets

[ahrens]

Presence of the kmem_cache walker prevents walk from running at all on userland targets (e.g. ztest / libzpool).

sdb> spa ztest | vdev 0 | member vdev_queue.vq_class | array | member vqc_pending_list | walk
sdb encountered an internal error due to a bug. Here's the
information you need to file the bug:
----------------------------------------------------------
Target Info:
	ProgramFlags.IS_LIVE
	Platform(<Architecture.X86_64: 1>, <PlatformFlags.IS_LITTLE_ENDIAN|IS_64_BIT: 3>)

Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/sdb/internal/repl.py", line 107, in eval_cmd
    for obj in invoke(self.target, [], input_):
  File "/usr/lib/python3/dist-packages/sdb/pipeline.py", line 166, in invoke
    yield from execute_pipeline(first_input, pipeline)
  File "/usr/lib/python3/dist-packages/sdb/pipeline.py", line 83, in execute_pipeline
    yield from massage_input_and_call(pipeline[-1], this_input)
  File "/usr/lib/python3/dist-packages/sdb/pipeline.py", line 43, in massage_input_and_call
    yield from cmd.call(objs)
  File "/usr/lib/python3/dist-packages/sdb/command.py", line 295, in call
    result, not issubclass(self.__class__, SingleInputCommand))
  File "/usr/lib/python3/dist-packages/sdb/command.py", line 256, in __invalid_memory_objects_check
    for obj in objs:
  File "/usr/lib/python3/dist-packages/sdb/command.py", line 564, in _call
    for type_, class_ in Walker.allWalkers.items()
  File "/usr/lib/python3/dist-packages/sdb/command.py", line 564, in <dictcomp>
    for type_, class_ in Walker.allWalkers.items()
  File "/usr/lib/python3/dist-packages/sdb/target.py", line 128, in type_canonicalize_name
    return type_canonical_name(prog.type(type_name))
LookupError: could not find 'struct kmem_cache'