osd/ReplicatedPG: fix user_version preservation for copy_from

osd/ReplicatedPG: fix user_version preservation for copy_from

In the process of fixing this for flush, we break promote, so we need to
adjust them both here.  Basic strategy: do not set user_modify, but handle
the user_version explicitly in the callbacks.

For copy_from, we don't have a clean way to pass the result through to
finish_copyfrom in do_osd_ops; do so by putting it in user_at_version. (If
we were to call finish_copyfrom directly from the callback this might
be simpler, but let's not go there right now.)

For promote, it is a trivial fix.

Signed-off-by: Sage Weil <sage@inktank.com>