Consolidate and clean up the first section of ip_output.c in light of the last year or two's work on routing:
Consolidate and clean up the first section of ip_output.c in light of the
last year or two's work on routing:

- Combine iproute initialization and flowtable lookup blocks, eliminating
  unnecessary tests for known-zero'd iproute fields.

- Add a comment indicating (a) why the route entry returned by the
  flowtable is considered stable and (b) that the flowtable lookup must
  occur after the setup of the mbuf flow ID.

- Assert the inpcb lock before any use of inpcb fields.

Reviewed by:	kmacy

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