perf: gc might try to populate the lookahead buffer each time

lfs.c

@@ -5225,7 +5225,7 @@ static int lfs_fs_gc_(lfs_t *lfs) {
     }
 
     // try to populate the lookahead buffer, unless it's already full
-    if (lfs->lookahead.size < 8*lfs->cfg->lookahead_size) {
+    if (lfs->lookahead.size < lfs_min(8 * lfs->cfg->lookahead_size, lfs->block_count)) {
         err = lfs_alloc_scan(lfs);
         if (err) {
             return err;