crimson/osd/objclass: Compilation warning

crimson/osd/objclass: Compilation warning

```
In copy constructor ‘ceph::buffer::v15_2_0::list::list(const ceph::buffer::v15_2_0::list&)’,
    inlined from ‘OSDOp::OSDOp(const OSDOp&)’ at ../src/osd/osd_types.h:4081:8,
    inlined from ‘int cls_cxx_snap_revert(cls_method_context_t, snapid_t)’ at ../src/crimson/osd/objclass.cc:279:37:
../src/include/buffer.h:945:20: warning: ‘op.OSDOp::indata.ceph::buffer::v15_2_0::list::_len’ is used uninitialized [-Wuninitialized]
  945 |         _len(other._len),
      |              ~~~~~~^~~~
../src/crimson/osd/objclass.cc: In function ‘int cls_cxx_snap_revert(cls_method_context_t, snapid_t)’:
../src/crimson/osd/objclass.cc:279:9: note: ‘op’ declared here
  279 |   OSDOp op{op = CEPH_OSD_OP_ROLLBACK};
      |
```

Signed-off-by: Matan Breizman <mbreizma@redhat.com>