librbd/cache: undefine dout_subsys before defining it

librbd/cache: undefine dout_subsys before defining it

otherwise we could have following warning from compiler
```
In file included from ../src/test/librbd/cache/test_mock_ReplicatedWriteLog.cc:42:
../src/librbd/cache/ImageWriteback.cc:13: warning: "dout_subsys" redefined
   13 | #define dout_subsys ceph_subsys_rbd
      |
In file included from ../src/test/librbd/cache/test_mock_ReplicatedWriteLog.cc:39:
../src/librbd/cache/ReplicatedWriteLog.cc:23: note: this is the location of the previous definition
   23 | #define dout_subsys ceph_subsys_rbd_rwl
      |
In file included from ../src/test/librbd/cache/test_mock_ReplicatedWriteLog.cc:43:
../src/librbd/cache/rwl/ImageCacheState.cc:12: warning: "dout_subsys" redefined
   12 | #define dout_subsys ceph_subsys_rbd_rwl
      |
In file included from ../src/test/librbd/cache/test_mock_ReplicatedWriteLog.cc:42:
../src/librbd/cache/ImageWriteback.cc:13: note: this is the location of the previous definition
   13 | #define dout_subsys ceph_subsys_rbd
      |
```

Signed-off-by: Kefu Chai <kchai@redhat.com>