fix: skip log files with deleted volumes in filer backup (#7692)

fix: skip log files with deleted volumes in filer backup (#3720)

When filer.backup or filer.meta.backup resumes after being stopped, it may
encounter persisted log files stored on volumes that have since been deleted
(via volume.deleteEmpty -force). Previously, this caused the backup to get
stuck in an infinite retry loop with 'volume X not found' errors.

This fix catches 'volume not found' errors when reading log files and skips
the problematic file instead of failing. The backup will now:
- Log a warning about the missing volume
- Skip the problematic log file
- Continue with the next log file, allowing progress

The VolumeNotFoundPattern regex was already defined but never used - this
change puts it to use.

Fixes #3720

weed/filer/stream.go

@@ -245,6 +245,7 @@ type ChunkStreamReader struct {
 
 var _ = io.ReadSeeker(&ChunkStreamReader{})
 var _ = io.ReaderAt(&ChunkStreamReader{})
+var _ = io.Closer(&ChunkStreamReader{})
 
 func doNewChunkStreamReader(ctx context.Context, lookupFileIdFn wdclient.LookupFileIdFunctionType, chunks []*filer_pb.FileChunk) *ChunkStreamReader {
 
@@ -403,8 +404,13 @@ func (c *ChunkStreamReader) fetchChunkToBuffer(chunkView *ChunkView) error {
 	return nil
 }
 
-func (c *ChunkStreamReader) Close() {
-	// TODO try to release and reuse buffer
+func (c *ChunkStreamReader) Close() error {
+	c.bufferLock.Lock()
+	defer c.bufferLock.Unlock()
+	c.buffer = nil
+	c.head = nil
+	c.chunkView = nil
+	return nil
 }
 
 func VolumeId(fileId string) string {