wgengine: plumb the PeerByKey from wgengine to magicsock

This was just added in 69f4b459 which doesn't yet use it. This still
doesn't yet use it. It just pushes it down deeper into magicsock where
it'll used later.

Updates #7617

Change-Id: If2f8fd380af150ffc763489e1ff4f8ca2899fac6
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

wgengine/userspace.go

@@ -347,6 +347,7 @@ func NewUserspaceEngine(logf logger.Logf, conf Config) (_ Engine, reterr error)
 		NetMon:           e.netMon,
 		ControlKnobs:     conf.ControlKnobs,
 		OnPortUpdate:     onPortUpdate,
+		PeerByKeyFunc:    e.PeerByKey,
 	}
 
 	var err error