rgw: use local error code in handle_individual_object()

op_ret is a member variable of RGWDeleteMultiObj shared between
concurrent callers of handle_individual_object(). because that coroutine
may suspend between writing to op_ret and later reading it back for
send_partial_response(), it's likely that other callers have modified it
in the meantime

store this error code in a local variable instead, so each coroutine has
its own consistent copy of it

Fixes: https://tracker.ceph.com/issues/76960

Signed-off-by: Casey Bodley <cbodley@redhat.com>