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This will be used for loading object map.  rbd_obj_read_sync() isn't
suitable because object map must be accessed through class methods.
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Reviewed-by: Dongsheng Yang <dongsheng.yang@easystack.cn>
Reviewed-by: Jeff Layton <jlayton@kernel.org>