fellow_cache: Generalize/refactor fellow_cache_seg_deref_locked()

src/fellow_cache.c

@@ -2920,24 +2920,33 @@ fellow_cache_seg_ref_locked(struct fellow_lru_chgbatch *lcb,
 	return (refcnt);
 }
 
-/* return new refcnt */
 static inline unsigned
-fellow_cache_seg_deref_locked(struct fellow_lru_chgbatch *lcb,
-    struct fellow_cache_seg *fcs)
+fellow_cache_seg_deref_n_locked(struct fellow_lru_chgbatch *lcb,
+    struct fellow_cache_seg *fcs, unsigned nref)
 {
 	unsigned refcnt;
 	int o, n;
 
-	AN(fcs->refcnt);
-	refcnt = --fcs->refcnt;
+	assert(fcs->refcnt >= nref);
+	fcs->refcnt -= nref;
+	refcnt = fcs->refcnt;
 
-	o = fellow_cache_shouldlru(fcs->state, fcs->fco->oc, refcnt + 1);
+	o = fellow_cache_shouldlru(fcs->state, fcs->fco->oc, refcnt + nref);
 	n = fellow_cache_shouldlru(fcs->state, fcs->fco->oc, refcnt);
 
 	fellow_cache_lru_chg(lcb, fcs, n - o);
 	return (refcnt);
 }
 
+/* return new refcnt */
+static inline unsigned
+fellow_cache_seg_deref_locked(struct fellow_lru_chgbatch *lcb,
+    struct fellow_cache_seg *fcs)
+{
+
+	return (fellow_cache_seg_deref_n_locked(lcb, fcs, 1));
+}
+
 static void
 fellow_cache_seg_unbusy(struct fellow_busy *fbo, struct fellow_cache_seg *fcs){
 	const enum fcos_state from = FCS_BUSY, to = FCS_WRITING;