Clarify allocation failure

src/buddy.h

@@ -595,6 +595,9 @@ typedef void buddy_pool_fill_f(struct buddy_reqs *, const void *);
 /*
  * i < 3: If both reqs are empty, at iteration 3 we
  * must hit the first filled alloc
+ *
+ * The XXXAN() (failed allocation) can trigger during load from
+ * fellow_log_memfail()
  */
 #define BUDDY_POOL_GET_FUNC(name, decl)			\
 decl struct buddy_reqs *					\
@@ -611,7 +614,7 @@ name ## _get(struct name *poolp, const void *priv)		\
 		CHECK_OBJ_NOTNULL(reqs, BUDDY_REQS_MAGIC);	\
 		if (buddy_reqs_next_ready(reqs))		\
 			return (reqs);				\
-		(void) BUDDYF(alloc_async_wait)(reqs);		\
+		XXXAN(BUDDYF(alloc_async_wait)(reqs));		\
 		if (buddy_reqs_next_ready(reqs))		\
 			return (reqs);				\
 		BUDDYF(alloc_wait_done)(reqs);			\