Fix compile warnings on 32-bit by using "0" instead of "(u64) NULL" to
assign to "c2_vq_req->reply_msg".
Signed-off-by: NRalf Thielow <ralf.thielow@googlemail.com>

[ Change from "(unsigned long) NULL" to plain old "0" as suggested by
  Bart Van Assche <bvanassche@acm.org>.  - Roland ]
Signed-off-by: NRoland Dreier <roland@purestorage.com>