Learnt/MIP nexthop is of different address family

Learnt/MIP nexthop is of different address family

Issue:
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Sporadic issue wherein learnt/MIP IPv4 address has IPv6 address as
nexthop and vice-versa in vpn-interfaces DS.

Solution:
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+ Added required check to make sure IPv4 MIP has IPv4 address as
  nexthop. Similarly for IPv6 MIPs.
+ This will ensure both local fixed IP and local MIPs pointing to
  same OF group in table-21.
+ Additionally simplified some part of the code in the same method by
  refactoring.

Change-Id: Ia94d0662187ed752a88eaedd9482f0539020e636
Signed-off-by: Somashekar Byrappa <somashekar.b@altencalsoftlabs.com>