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