hyperv/hn: Implement LRO

- Implement the LRO using tcp_lro APIs, and LRO is enabled by default.
- Add several stats sysctl nodes.
- Check IP/TCP length before sending the packet to tcp_lro_rx(), if host
  does not provide RX csum information (*); and add an option through
  sysctl to always trust host TCP segment csum checks (default is off).
- Add sysctl to control the LRO entry depth; it is disabled by default.
  It is used to avoid holding too much TCP segments in driver.  Limiting
  the LRO entry depth helps a lot in a one/two streams RX test.

This one 3x the RX performance on my local test (3Gbps -> 10Gbps), and
~2x the RX performance over a directly connected 40Ge network (5Gbps ->
9Gbps).

(*) It seems the host stops supplying csum information, once the network
load is high.  This still needs investigation...

Reviewed by:		Hongjiang Zhang <honzhan microsoft com>,
			Dexuan Cui <decui microsoft com>,
			Jun Su <junsu microsoft com>,
			delphij
Tested by:		me (local),
			Hongjiang Zhang <honzhan microsoft com>
			(directly connected 40Ge)
Approved by:		delphij (mentor), adrian (mentor, no objection)
With feedback from:	delphij, Hongjiang Zhang <honzhan microsoft com>
Sponsored by:		Microsoft OSTC
Differential Revision:	https://reviews.freebsd.org/D4824

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