ofproto/nx_match: various minor clean up
ofproto/nx_match: various minor clean up

- initialize Flow.tun_id
- avoid name clash with _set_nxm_headers
- eliminated duplicated MFTPSRC
- s/len/len_/ to avoid name clash with builtin len()

Signed-off-by: Isaku Yamahata <4b3abd66323411eeceff090ab3754dd766fc4b25@valinux.co.jp>
Signed-off-by: FUJITA Tomonori <93dac1fe9c4b2a3957982200319981492ad4976e@lab.ntt.co.jp>

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