librbd/cache/pwl: initialize generate_test_instances() objects
... to prevent check-generated.sh failures such as:
**** librbd::cache::pwl::WriteLogPoolRoot test 1 dump_json check failed ****
ceph-dencoder type librbd::cache::pwl::WriteLogPoolRoot select_test 1 dump_json > /tmp/typ-cAoWrqlHC
ceph-dencoder type librbd::cache::pwl::WriteLogPoolRoot select_test 1 encode decode dump_json > /tmp/typ-ES5yHpfGL
5c5
< "flushed_sync_gen": 0,
---
> "flushed_sync_gen": 255,
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
(cherry picked from commit
2c131f57d63454de39210375ce75a282df6fe365)