osd: clean up src_oid, src_obc map key calculation

osd: clean up src_oid, src_obc map key calculation

Be consistent about how we generate the src_oid and src_oloc, so that we
feed good value into find_object_context and use a consistent key for
the src_obc map<>.  This fixes a crash in do_osd_ops() due to a missing
src_obc key when the get_src_oloc() normalizes the key in do_op() but not
in do_osd_ops().

Also use a nicer name.

Fixes: #1897
Signed-off-by: Sage Weil <sage@newdream.net>