fixup! refactor disk.Cache to allow concurrent inserts

add back correct Reserve check (we can't evict enough items if
c.reservedSize+size > c.maxSize)

cache/disk/lru.go

@@ -130,7 +130,7 @@ func (c *sizedLRU) MaxSize() int64 {
 }
 
 func (c *sizedLRU) Reserve(size int64) bool {
-	if size < 0 || size > c.maxSize || c.currentSize+size > c.maxSize {
+	if size < 0 || size > c.maxSize || c.reservedSize+size > c.maxSize {
 		return false
 	}