wgengine/netstack: fix shouldProcessInbound peerapi non-SYN handling

It was eating TCP packets to peerapi ports to subnet routers.  Some of
the TCP flow's packets went onward, some got eaten.  So some TCP flows
to subnet routers, if they used an unfortunate TCP port number, got
broken.

Change-Id: Ifea036119ccfb081f4dfa18b892373416a5239f8
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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