Keep track of free dlechg structs

src/fellow_log_dle_chg.h

@@ -147,7 +147,8 @@ dlechg_pool_fill(struct buddy_reqs *reqs, const void *priv)
 struct fellow_dlechg_top {
 	unsigned			magic;
 #define FELLOW_DLECHG_TOP_MAGIC		0x710e71a2
-	buddy_t			*buddy;
+	unsigned			nfree;	// mostly debug aid
+	buddy_t				*buddy;
 	struct fellow_dlechg_head	head;
 	VSLIST_HEAD(,fellow_dlechg)	free;
 	struct dlechg_pool		pool[1];
@@ -160,8 +161,11 @@ fellow_dlechg_alloc(struct fellow_dlechg_top *fdct)
 	struct buddy_ptr_extent mem;
 
 	fdc = VSLIST_FIRST(&fdct->free);
-	if (fdc != NULL)
+	if (fdc != NULL) {
 		VSLIST_REMOVE_HEAD(&fdct->free, u.freelist);
+		AN(fdct->nfree);
+		fdct->nfree--;
+	}
 	else {
 		mem = buddy_get_next_ptr_extent(
 		    dlechg_pool_get(fdct->pool, NULL));
@@ -296,5 +300,6 @@ fellow_dlechg_get(struct fellow_dlechg_top *fdct, struct fellow_dle *e)
 
 	memset(fdc, 0, sizeof *fdc);
 	VSLIST_INSERT_HEAD(&fdct->free, fdc, u.freelist);
+	fdct->nfree++;
 	return (e);
 }