ANDROID: Incremental fs: Fix incfs to work on virtio-9p

Test: incfs_test on a virtio 9p drive. Note test 6 fails still,
but I think this is a result of caching directory entries over a
network file system.
Bug: 161802292
Signed-off-by: Paul Lawrence <paullawrence@google.com>
Change-Id: I6986fb3e9b403181cf81024046f394960caf4620