Add OpenMmapFileWithSize method (#6753)

Signed-off-by: Ganesh Vernekar <cs15btech11018@iith.ac.in>

tsdb/fileutil/mmap.go

@@ -25,16 +25,23 @@ type MmapFile struct {
 }
 
 func OpenMmapFile(path string) (*MmapFile, error) {
+	return OpenMmapFileWithSize(path, 0)
+}
+
+func OpenMmapFileWithSize(path string, size int) (*MmapFile, error) {
 	f, err := os.Open(path)
 	if err != nil {
 		return nil, errors.Wrap(err, "try lock file")
 	}
+	if size <= 0 {
 		info, err := f.Stat()
 		if err != nil {
 			return nil, errors.Wrap(err, "stat")
 		}
+		size = int(info.Size())
+	}
 
-	b, err := mmap(f, int(info.Size()))
+	b, err := mmap(f, size)
 	if err != nil {
 		return nil, errors.Wrap(err, "mmap")
 	}