lrdp-v2: vmalloc: use new ListWalker constructor

ListWalker.__init__ was recently changed to remove some unneeded
arguments. The vmalloc parser was also updated at roughly the same time
with a new usage of the ListWalker class and was using the old
constructor. Update it.

Change-Id: Ia47c22511aab7207b65813d25c9f97d65c24e2db