Fix list handling to only use the API

Remove all instances of list handling where the API is not used
and instead list data members are directly accessed. Doing this
sort of thing is bad for portability.

Additionally, ensure that list_link_init() is called on newly
created list nodes. This ensures the node is properly initialized
and does not rely on the assumption that zero'ing the list_node_t
via kmem_zalloc() is the same as proper initialization.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>