cmd/tailscale,ipn,net/netutil: remove rp_filter strict mode warnings (#18863)

PR #18860 adds firewall rules in the mangle table to save outbound packet
marks to conntrack and restore them on reply packets before the routing
decision. When reply packets have their marks restored, the kernel uses
the correct routing table (based on the mark) and the packets pass the
rp_filter check.

This makes the risk check and reverse path filtering warnings unnecessary.

Updates #3310
Fixes tailscale/corp#37846

Signed-off-by: Mike O'Driscoll <mikeo@tailscale.com>

cmd/tailscaled/depaware-min.txt

@@ -64,7 +64,7 @@ tailscale.com/cmd/tailscaled dependencies: (generated by github.com/tailscale/de
         tailscale.com/feature/condregister/portmapper                from tailscale.com/feature/condregister
         tailscale.com/feature/condregister/useproxy                  from tailscale.com/feature/condregister
         tailscale.com/health                                         from tailscale.com/control/controlclient+
-        tailscale.com/health/healthmsg                               from tailscale.com/ipn/ipnlocal+
+        tailscale.com/health/healthmsg                               from tailscale.com/ipn/ipnlocal
         tailscale.com/hostinfo                                       from tailscale.com/cmd/tailscaled+
         tailscale.com/ipn                                            from tailscale.com/cmd/tailscaled+
         tailscale.com/ipn/conffile                                   from tailscale.com/cmd/tailscaled+

cmd/tailscaled/depaware.txt

@@ -309,7 +309,7 @@ tailscale.com/cmd/tailscaled dependencies: (generated by github.com/tailscale/de
         tailscale.com/feature/useproxy                               from tailscale.com/feature/condregister/useproxy
         tailscale.com/feature/wakeonlan                              from tailscale.com/feature/condregister
         tailscale.com/health                                         from tailscale.com/control/controlclient+
-        tailscale.com/health/healthmsg                               from tailscale.com/ipn/ipnlocal+
+        tailscale.com/health/healthmsg                               from tailscale.com/ipn/ipnlocal
         tailscale.com/hostinfo                                       from tailscale.com/client/web+
         tailscale.com/ipn                                            from tailscale.com/client/local+
    W    tailscale.com/ipn/auditlog                                   from tailscale.com/cmd/tailscaled