drm/i915: Simplify the flush code

Reduce differences with Linux 3.8.13
inpcb: Factor out in_pcbresetroute()
udp: Use ncpus2 instead of ncpus, since udp uses ncpus2 netisrs

There is actually no functional change.
inpcb: Factor out ASSERT_INP_NOTINHASH
udp: Add NOTINHASH assertion to udp_inswildcardhash()

So that we could catch programming bugs earlier.