Start getting the disk log block reserve earlier

Ref #28

src/fellow_log.c

@@ -2286,11 +2286,12 @@ logbuffer_alloc_some(struct fellow_logbuffer *lbuf,
 	 * also allocate that async to not delay the happy
 	 * path (we are in the middle of flushing logs under the lock)
 	 *
-	 * XXX is 20% a good measure?
+	 * XXX is 25% a good measure?
 	 */
 	if (lbuf->dskreqs == NULL &&
 	    lbuf->dskreqs_mem.reqs.magic == 0 &&
-	    lbuf->logreg->free_n * 5 < lbuf->logreg->space) {
+	    (lbuf->logreg->free_n * 4 < lbuf->logreg->space ||
+	     lbuf->logreg->free_n < LBUF_DSK_RSV_REQS * 4)) {
 		BUDDY_REQS_INIT(&lbuf->dskreqs_mem, lbuf->membuddy);
 		BUDDY_REQS_PRI(&lbuf->dskreqs_mem.reqs, FEP_MEM_LOG);
 		AN(buddy_req_extent(&lbuf->dskreqs_mem.reqs,