mvcc/backend: enable 'NoFreelistSync' by default (linux)

Signed-off-by: Gyu-Ho Lee <gyuhox@gmail.com>

mvcc/backend/config_linux.go

@@ -27,7 +27,8 @@ import (
 // (https://github.com/torvalds/linux/releases/tag/v2.6.23), mmap might
 // silently ignore this flag. Please update your kernel to prevent this.
 var boltOpenOptions = &bolt.Options{
-	MmapFlags: syscall.MAP_POPULATE,
+	MmapFlags:      syscall.MAP_POPULATE,
+	NoFreelistSync: true,
 }
 
 func (bcfg *BackendConfig) mmapSize() int { return int(bcfg.MmapSize) }