network: Add IgnoreDdontFragment= option for Fragmentation control (#28131)

From `ip-link(8)`:
> [no]ignore-df - enables/disables IPv4 DF
                      suppression on this tunnel.  Normally datagrams
                      that exceed the MTU will be fragmented; the
                      presence of the DF flag inhibits this, resulting
                      instead in an ICMP Unreachable (Fragmentation
                      Required) message.  Enabling this attribute causes
                      the DF flag to be ignored.

If this option is enabled for a GRE/GRETAP tunnel, the `DF` flag in the outer IP header
will not inherit the inner IP header's `DF` flag.

This is useful to transfer packets that exceed the MTU of the underlay
network.

src/network/netdev/tunnel.h

@@ -54,7 +54,8 @@ typedef struct Tunnel {
         Ip6TnlMode ip6tnl_mode;
         FooOverUDPEncapType fou_encap_type;
 
-        bool pmtudisc;
+        int pmtudisc;
+        bool ignore_df;
         bool copy_dscp;
         bool independent;
         bool fou_tunnel;