rbd: stop copying num_osd_ops in rbd_obj_issue_copyup()

In preparation for deep-flatten feature, stop copying num_osd_ops from
the original request in rbd_obj_issue_copyup().  Split the calculation
into count_{write,zeroout}_ops() respectively and determine whether the
assert_exists guard is needed with the new rbd_obj_copyup_enabled().

As a nice side effect, we no longer guard in the writefull case as the
copyup'ed object is always fully overwritten.

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