scylla-gdb.py: intrusive_slist: avoid infinite recursion in __len__()
scylla-gdb.py: intrusive_slist: avoid infinite recursion in __len__()

Said method currently uses a list() to iterate over all elements,
determining the length. Passing `self` to `list()` will however make
call `len()` first, causing infinite recursion.

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla