librbd/object_map: don't resize object map in handle_load_object_map()

librbd/object_map: don't resize object map in handle_load_object_map()

Currently it's done in two cases:

- if the loaded object map is larger than expected based on byte size,
  it's truncated to expected number of objects
- in case of deep-copy, if the loaded object map is smaller than diff
  state, it's expanded to get "track the largest of all versions in the
  set" semantics

Both of these cases can be easily dealt with without modifying the
object map.  Being able to process a const object map is needed for
working on in-memory object map which is external to DiffRequest.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>