xnbd_mq: Fix for_each_segment loop

Since kernel 3.14 bvec is not a pointer

Signed-off-by: Sagi Grimberg <sagig@mellanox.com>
BACKPROTS: compilation error for kernel > 3.14

for_each_segment loop

Signed-off-by: Sagi Grimberg <sagig@mellanox.com>
autogen: Add backports facilities

Signed-off-by: Sagi Grimberg <sagig@mellanox.com>