Call fdatasync/fsync after writing out non-meta pages

This avoids a case where writes can be reordered so meta page is
written before a page it refers to, potentially causing a corrupt
database after a power loss or kernel crash.

sync_linux.go

@@ -0,0 +1,10 @@
+package bolt
+
+import (
+	"os"
+	"syscall"
+)
+
+func fdatasync(f *os.File) error {
+	return syscall.Fdatasync(int(f.Fd()))
+}

sync_std.go

@@ -0,0 +1,10 @@
+// +build !linux
+
+package bolt
+
+import "os"
+
+// Fall back to syncing metadata too.
+func fdatasync(f *os.File) error {
+	return f.Sync()
+}

tx.go

@@ -329,6 +329,9 @@ func (t *Tx) write() error {
 			return err
 		}
 	}
+	if err := fdatasync(t.db.file); err != nil {
+		return err
+	}
 
 	// Clear out page cache.
 	t.pages = make(map[pgid]*page)