fellow_cache: support freeing an fcs which contains itself

src/fellow_cache.c

@@ -1406,6 +1406,7 @@ static void
 fellow_cache_seg_free(struct buddy_returns *memret,
     struct fellow_cache_seg *fcs, unsigned deref)
 {
+	struct buddy_ptr_extent mem;
 
 	CHECK_OBJ_NOTNULL(fcs, FELLOW_CACHE_SEG_MAGIC);
 
@@ -1436,8 +1437,7 @@ fellow_cache_seg_free(struct buddy_returns *memret,
 		    FCS_INCORE, FCS_DISK);
 		fellow_cache_lru_chgbatch_apply(lcb);
 	}
-	if (fcs->alloc.ptr)
-		AN(buddy_return_ptr_extent(memret, &fcs->alloc));
+	TAKE(mem, fcs->alloc);
 	AZ(fcs->fcs_onlru);
 	assert(fcs->refcnt == deref);
 	fcs->refcnt = 0;
@@ -1446,6 +1446,8 @@ fellow_cache_seg_free(struct buddy_returns *memret,
 	 * at this point, the fcs is not consistent in all cases, e.g. FCS_EVICT
 	 * has no memory - but this is the point where it does no longer exist
 	 */
+	if (mem.ptr)
+		AN(buddy_return_ptr_extent(memret, &mem));
 }
 
 /*