[neighsync] VXLAN EVPN neighbors not in NEIGH_TABLE (PR sonic-net#3478)

VXLAN EVPN learned routes are not entered into NEIGH_TABLE as per
Issue sonic-net#3384.

The EVPN VXLAN HLD specifically states this should be populated so it triggers
an update to the SAI database:

https://github.com/sonic-net/SONiC/blob/master/doc/vxlan/EVPN/EVPN_VXLAN_HLD.md#438-mac-ip-route-handling

The reason it was not occurring is NOARP entries were being rejected, this
patch adds an exception for externally learned neighbors.

Signed-off-by: Brad House (@bradh352)

Author: bradh352

neighsyncd/neighsync.cpp

@@ -16,6 +16,11 @@
 #include <algorithm>
 #include <linux/neighbour.h>
 
+#ifndef NTF_EXT_LEARNED
+/* from include/uapi/linux/neighbour.h */
+#  define NTF_EXT_LEARNED (1 << 4)
+#endif
+
 using namespace std;
 using namespace swss;