fetch: Atomically capture stale_oc state

It should be safe as it was because there shouldn't be a transition from
BOS_FINISHED to BOS_FAILED, but now that it can be atomic, it should not
only be safe but also future-proof. It originally matched ignore_state
from the coccinelle patch because of the separate boc variable, and
had to be manually edited.

Author: dridi

bin/varnishd/cache/cache_fetch.c

@@ -886,8 +886,7 @@ vbf_stp_condfetch(struct worker *wrk, struct busyobj *bo)
 		VSLb(bo->vsl, SLT_Notice,
 		    "vsl: Conditional fetch wait for streaming object");
 		/* XXX: We should have a VCL controlled timeout here */
-		(void)ObjWaitState(stale_oc, BOS_FINISHED);
-		stale_state = stale_boc->state;
+		stale_state = ObjWaitState(stale_oc, BOS_FINISHED);
 		HSH_DerefBoc(bo->wrk, stale_oc);
 		stale_boc = NULL;
 		if (stale_state != BOS_FINISHED) {