rbd: silence bogus uninitialized warning in rbd_object_map_update_finish()

Some versions of gcc (so far 6.3 and 7.4) throw a warning:

  drivers/block/rbd.c: In function 'rbd_object_map_callback':
  drivers/block/rbd.c:2124:21: warning: 'current_state' may be used uninitialized in this function [-Wmaybe-uninitialized]
        (current_state == OBJECT_EXISTS && state == OBJECT_EXISTS_CLEAN))
  drivers/block/rbd.c:2092:23: note: 'current_state' was declared here
    u8 state, new_state, current_state;
                          ^~~~~~~~~~~~~

It's bogus because all current_state accesses are guarded by
has_current_state.

Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Reviewed-by: Dongsheng Yang <dongsheng.yang@easystack.cn>