HashIndex: fix list_by_hash handling of next->is_max()

HashIndex: fix list_by_hash handling of next->is_max()

get_path_str() should not handle hobject_t::get_max().  get_path_str()
now asserts that the passed object is not max and the callers now check
for is_max().  This caused HashIndex.cc to incorrectly scan an entire
collection before returning no objects rather than scanning the top
level and returning no objects.  It did not actually list_by_hash to
return an incorrect answer, however.

Signed-off-by: Samuel Just <sam.just@inktank.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>