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.

test/test-network/conf/25-erspan0-tunnel.netdev

@@ -13,3 +13,4 @@ Local = 172.16.1.200
 Remote = 172.16.1.100
 Key=101
 SerializeTunneledPackets=true
+IgnoreDontFragment=true

test/test-network/conf/25-gretap-tunnel.netdev

@@ -8,3 +8,4 @@ Local=10.65.223.238
 Remote=10.65.223.239
 Key=106
 SerializeTunneledPackets=true
+IgnoreDontFragment=true

test/test-network/systemd-networkd-tests.py

@@ -1796,6 +1796,8 @@ class NetworkdNetDevTests(unittest.TestCase, Utilities):
         self.assertRegex(output, 'okey 0.0.0.106')
         self.assertRegex(output, 'iseq')
         self.assertRegex(output, 'oseq')
+        self.assertIn('nopmtudisc', output)
+        self.assertIn('ignore-df', output)
         output = check_output('ip -d link show gretap98')
         print(output)
         self.assertRegex(output, 'gretap remote 10.65.223.239 local any dev dummy98')
@@ -1955,6 +1957,8 @@ class NetworkdNetDevTests(unittest.TestCase, Utilities):
         self.assertNotIn('erspan_hwid 1f', output)
         self.assertIn('ikey 0.0.0.101', output)
         self.assertIn('iseq', output)
+        self.assertIn('nopmtudisc', output)
+        self.assertIn('ignore-df', output)
         output = check_output('ip -d link show erspan98')
         print(output)
         self.assertIn('erspan remote 172.16.1.100 local any', output)