librbd/cache/pwl: initialize generate_test_instances() objects

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)

Conflicts:
	src/librbd/cache/pwl/Types.cc [ commit 6eb14774fec0 ("librbd:
	  build without "using namespace std"") not in pacific ]
	src/librbd/cache/pwl/ssd/Types.h [ ditto ]