Fix LBUF_DSKPOOL_SIZE

forgot some members in the calculation, so on SmartOS we hit
an assertion that the resulting pool is below 4KB

src/fellow_log.c

@@ -412,7 +412,8 @@ assert_seq_macros(void)
 
 // <4K to fit in logblk
 #define LBUF_DSKPOOL_SIZE					\
-	(((1<<12) - 2 * sizeof(struct buddy_reqs)) /	\
+	(((1<<12) - sizeof(void *) - 2 * (			\
+	  sizeof(struct buddy_reqs) + sizeof(unsigned))) /	\
 	(2 * sizeof(struct i_reqalloc)))
 #define LOGBLK_MEMPOOL_SIZE 2
 //lint -e{506, 527}