zebra: check router_flag change for neigh update

zebra: check router_flag change for neigh update

Neigh update can have router_flag change, from unset to set and
viceversa. This is the case where MAC, IP and VLAN are same but
entry's flag moved from R to not R bit and reverse case.

Router flag change needs to trigger bgpd to inform all evpn peers
to remove from the evpn route.

Testing Done:
Send GARP with and without R bit from host and validate neigh entry
and evpn neigh and  mac-ip route entry in zebra and bgpd.

Check Peer VTEP evpn route entry where router flag is (un)set.
With R-bit

Route [2]:[0]:[0]:[48]:[00:1f:2f:db:45:a6]:[128]:[2006:33:33:2::10]
VNI 1001
Imported from
27.0.0.16:5:[2]:[0]:[0]:[48]:[00:1f:2f:db:45:a6]:[128]:[2006:33:33:2::10]
   4435 5551
27.0.0.16 from MSP1(uplink-1) (27.0.0.9)
   Origin IGP, valid, external, bestpath-from-AS 4435, best
   Extended Community: RT:5551:1001 ET:8 ND:Router
   Flag
   AddPath ID: RX 0, TX 1261
   Last update: Wed Aug 15 20:52:14 2018

Without R-bit

Route [2]:[0]:[0]:[48]:[00:1f:2f:db:45:a6]:[128]:[2006:33:33:2::10]
VNI 1001
Imported from
27.0.0.16:5:[2]:[0]:[0]:[48]:[00:1f:2f:db:45:a6]:[128]:[2006:33:33:2::10]
   4435 5551
27.0.0.16 from MSP2(uplink-2) (27.0.0.10)
   Origin IGP, valid, external, bestpath-from-AS 4435, best
   Extended Community: RT:5551:1001 ET:8
   AddPath ID: RX 0, TX 1263
   Last update: Wed Aug 15 20:53:10 2018

Signed-off-by: Chirag Shah <chirag@cumulusnetworks.com>