ANDROID: dm-bow: Pass through zero sized requests

Zero sized requests appeared in 5.15, and break validation logic in user
space. Simply pass through, as there is no reason to handle.

aosp/3238494 enables checkpoints on ext4 in cuttlefish, get this too

With all the patches in place, run:

launch_cvd \
  -userdata_format="ext4" \
  --data_policy=always_create --blank_data_image_mb=16000

then run

atest vts_kernel_checkpoint_test

Test: atest vts_kernel_checkpoint_test passes
Bug: 210958368
Bug: 379587919
Signed-off-by: Paul Lawrence <paullawrence@google.com>
Change-Id: I3bc8037dc53e02f10c3f41de2f1176e774482815