librbd: clean up object map update interface, revisited

librbd: clean up object map update interface, revisited

In master, the "batch update" change [1] was merged before the "order
concurrent updates" [2], while in jewel the latter is already
backported [3]. A backport of [1] to jewel was attempted, and was
necessarily applied on top of [3] - i.e. in the reverse order compared
to how the commits went into master. This reverse ordering caused the
automated cherry-pick to miss some parts of [1] which this commit is
adding manually.

[1] https://github.com/ceph/ceph/pull/11510
[2] https://github.com/ceph/ceph/pull/12420
[3] https://github.com/ceph/ceph/pull/12909

Signed-off-by: Mykola Golub <mgolub@mirantis.com>
Signed-off-by: Nathan Cutler <ncutler@suse.com>