I can reproduce the issue now, and it is easy to trigger it via your test
code on scsi device, but a bit difficult to get it on null_blk.

Turns out it is a block core issue, and it is in bio_will_gap() which gets
the last bvec via 'bi_io_vec[prev->bi_vcnt - 1]' directly.  I have posted
out one patchset for fixing the issue:

http://marc.info/?l=linux-kernel&m=145551975429092&w=2

Thanks Ming!

I'll give it a shot...
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