lib/file: don't run preallocate concurrently

This seems to cause file systems to get the amount of free space
wrong.

lib/file/preallocate_windows.go

@@ -4,6 +4,7 @@ package file
 
 import (
 	"os"
+	"sync"
 	"syscall"
 	"unsafe"
 
@@ -15,6 +16,7 @@ var (
 	ntdll                        = windows.NewLazySystemDLL("ntdll.dll")
 	ntQueryVolumeInformationFile = ntdll.NewProc("NtQueryVolumeInformationFile")
 	ntSetInformationFile         = ntdll.NewProc("NtSetInformationFile")
+	preAllocateMu                sync.Mutex
 )
 
 type fileAllocationInformation struct {
@@ -42,6 +44,9 @@ func PreAllocate(size int64, out *os.File) error {
 		return nil
 	}
 
+	preAllocateMu.Lock()
+	defer preAllocateMu.Unlock()
+
 	var (
 		iosb       ioStatusBlock
 		fsSizeInfo fileFsSizeInformation