osd/ReplicatedPG: use get_object_context in trim_object

find_object_context() has all the logic to choose a particular clone given
a logical snap.  In the trim case, we want none of that: we just need to
pull the obc for a specific clone instance.  Note that this changes
none of the failure cases (previous we asserted r == 0).

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