ANDROID: virtio_blk: Remove BUG_ON for discard/zero ops

The virtio_blk driver used by cuttlefish can crash if discard/zero ops,
added in 4.20, pass through virtio_queue_rq(). Our theory is that this
happens because multiple discards are merged, causing nr_phys_segments
to be increased beyond the sg_elem size. This makes sense as a
discard/zero is a form of I/O not backed by specific data.

This patch should be submitted upstream once the true root cause has
been tracked down and full analysis of the issue has been done.

Originally-by: Satya Tangirala <satyat@google.com>
Bug: 144592287
Test: boot cuttlefish with f2fs userdata and wait 1 minute
Change-Id: Ic73e08035c91ec9c718e6496930b326d084062a4
Signed-off-by: Alistair Delva <adelva@google.com>