librbd: try to preserve object map for diff-iterate in fast-diff mode

librbd: try to preserve object map for diff-iterate in fast-diff mode

As an optimization, try to ensure that the object map for the end
version is preloaded through the acquisition of exclusive lock and
as a consequence remains around until exclusive lock is released.
If it's not around, DiffRequest would (re)load it on each call.

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