vknulldrv: Use a default allocator when none provided
am: d3b1450e66

* commit 'd3b1450e6606cacff0d2c6e0ff6e45f1420ad5ee':
  vknulldrv: Use a default allocator when none provided

Change-Id: Ia6284f0d68114dc8f3e6ec5257ba4de05d0b71d6