lib/file: retry preallocate on EINTR

Before this change, sometimes preallocate failed with EINTR which
rclone ignored.

Retrying the syscall is the correct thing to do and seems to make
preallocate 100% reliable.

lib/file/preallocate_unix.go

@@ -6,6 +6,7 @@ import (
 	"os"
 	"sync"
 	"sync/atomic"
+	"syscall"
 
 	"github.com/rclone/rclone/fs"
 	"golang.org/x/sys/unix"
@@ -25,19 +26,23 @@ var (
 const PreallocateImplemented = true
 
 // PreAllocate the file for performance reasons
-func PreAllocate(size int64, out *os.File) error {
+func PreAllocate(size int64, out *os.File) (err error) {
 	if size <= 0 {
 		return nil
 	}
+
 	preAllocateMu.Lock()
 	defer preAllocateMu.Unlock()
+
+	for {
+
 		index := atomic.LoadInt32(&fallocFlagsIndex)
 	again:
 		if index >= int32(len(fallocFlags)) {
 			return nil // Fallocate is disabled
 		}
 		flags := fallocFlags[index]
-	err := unix.Fallocate(int(out.Fd()), flags, 0, size)
+		err = unix.Fallocate(int(out.Fd()), flags, 0, size)
 		if err == unix.ENOTSUP {
 			// Try the next flags combination
 			index++
@@ -50,6 +55,10 @@ again:
 		if err == unix.ENOSPC {
 			return ErrDiskFull
 		}
+		if err != syscall.EINTR {
+			break
+		}
+	}
 	return err
 }