multilist / spl_list doesn't work on libzpool

[ahrens]

when debugging a ztest process:

sdb> addr ARC_mru | member arcs_list[1] | multilist
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 85, 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 66, in massage_input_and_call
    yield from cmd.call(objs)
  File "/usr/lib/python3/dist-packages/sdb/command.py", line 291, in call
    result, not issubclass(self.__class__, SingleInputCommand))
  File "/usr/lib/python3/dist-packages/sdb/command.py", line 252, in __invalid_memory_objects_check
    for obj in objs:
  File "/usr/lib/python3/dist-packages/sdb/command.py", line 612, in _call
    yield from self.walk(obj)
  File "/usr/lib/python3/dist-packages/sdb/commands/spl/multilist.py", line 33, in walk
    yield from sdb.execute_pipeline([sublist], [SPLList()])
  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 66, in massage_input_and_call
    yield from cmd.call(objs)
  File "/usr/lib/python3/dist-packages/sdb/command.py", line 291, in call
    result, not issubclass(self.__class__, SingleInputCommand))
  File "/usr/lib/python3/dist-packages/sdb/command.py", line 252, in __invalid_memory_objects_check
    for obj in objs:
  File "/usr/lib/python3/dist-packages/sdb/command.py", line 612, in _call
    yield from self.walk(obj)
  File "/usr/lib/python3/dist-packages/sdb/commands/spl/spl_list.py", line 32, in walk
    node = first_node.next
AttributeError: 'struct list_node' has no member 'next'
----------------------------------------------------------
Link: https://github.com/delphix/sdb/issues/new

Indeed, in libspl/libzfs, struct list_node does not have next:
lib/libspl/include/sys/list_impl.h:

struct list_node {
	struct list_node *list_next;
	struct list_node *list_prev;
};

Seems like the root of the problem is the difference between libzpool and zfs.ko. Ideally, sdb would have target-specific functionality (i.e. user vs kernel). In the absence of that, we might want to change libspl's list_node to mirror the linux kernel's