Flip ovn_emit_need_to_frag to True by default

Setting it to False is relevant only for older kernels.

Major distros ship the necessary kernel for a while, e.g. Ubuntu LTS has
5.2 since 20.04.0. RHEL ships 5.14.0 (+backports) since 9.0.

Related: https://bugzilla.redhat.com/show_bug.cgi?id=2314429

Change-Id: I52124fccf800ecd64dfb1a2f40897dea24cd62d7

neutron/conf/plugins/ml2/drivers/ovn/ovn_conf.py

@@ -175,12 +175,12 @@ ovn_opts = [
                        "option to be unset globally.\n"
                        "See the ovn-nb(5) man page for available options.")),
     cfg.BoolOpt('ovn_emit_need_to_frag',
-                default=False,
+                default=True,
                 help=_('Configure OVN to emit "need to frag" packets in '
                        'case of MTU mismatches.\n'
-                       'Before enabling this option make sure that '
+                       'You may have to disable this option if you are '
-                       'it is supported by the host kernel (version >= 5.2) '
+                       'running an old host kernel (version < 5.2). You may '
-                       'or by checking the output of the following command:\n'
+                       'check the output of the following command:\n'
                        'ovs-appctl -t ovs-vswitchd dpif/show-dp-features '
                        'br-int | grep "Check pkt length action".')),
     cfg.BoolOpt('disable_ovn_dhcp_for_baremetal_ports',

releasenotes/notes/enable-ovn_emit_need_to_frag-by-default-9ba0169e70a55173.yaml

@@ -0,0 +1,7 @@
+---
+upgrade:
+  - |
+    The ``ovn_emit_need_to_frag`` option for OVN is now enabled by default.
+    This may result in performance issues on older kernels (older than 5.2). If
+    you are still using an old kernel, you may consider setting it to
+    ``False``.