vfs: Add exponential backoff during ENOSPC retries

Add an exponentially increasing delay during retries up ENOSPC error
to avoid exhausting the 10 retries too soon when the cache space
recovery from item resets is not available from the file system yet
or consumed by other large cache writes.

vfs/vfscache/cache.go

@@ -610,7 +610,6 @@ func (c *Cache) clean(removeCleanFiles bool) {
 	if os.IsNotExist(err) {
 		return
 	}
-
 	c.mu.Lock()
 	oldItems, oldUsed := len(c.item), fs.SizeSuffix(c.used)
 	c.mu.Unlock()