vfs: fix incorrect modtime on fs which don't support setting modtime

Before this change we were using the Precision literally to round the
precision of the mod times.

However fs.ModTimeNotSupported is 100y on backends which don't support
setting modtimes so rounding to 100y was producing very strange
results.

See: https://forum.rclone.org/t/saving-files-causes-wrong-modified-time-to-be-set-for-a-few-seconds-on-webdav-mount-with-bitrix24/36451/

vfs/file.go

@@ -296,6 +296,9 @@ func (f *File) activeWriters() int {
 // It should be called with the lock held
 func (f *File) _roundModTime(modTime time.Time) time.Time {
 	precision := f.d.f.Precision()
+	if precision == fs.ModTimeNotSupported {
+		return modTime
+	}
 	return modTime.Truncate(precision)
 }