net/{batching,udprelay},wgengine/magicsock: add SO_RXQ_OVFL clientmetrics

For the purpose of improved observability of UDP socket receive buffer
overflows on Linux.

Updates tailscale/corp#37679

Signed-off-by: Jordan Whited <jordan@tailscale.com>

net/batching/conn_default.go

@@ -10,7 +10,7 @@ import (
 )
 
 // TryUpgradeToConn is no-op on all platforms except linux.
-func TryUpgradeToConn(pconn nettype.PacketConn, _ string, _ int) nettype.PacketConn {
+func TryUpgradeToConn(pconn nettype.PacketConn, _ string, _ int, _ string) nettype.PacketConn {
 	return pconn
 }