rgw: don't hold data_lock over frontend io

rgw: don't hold data_lock over frontend io

get_obj_data::data_lock protects access to the read_list, which the
librados finisher thread appends to on completions

locking isn't needed over calls to d->client_cb->handle_data() in
get_obj_iterate_cb(), because this callback is called synchronously
from the frontend/process_request() thread - no other thread will
be submitting io to the d->client_cb callback

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