Zebra: Fix setting source for 5549-learnt routes via ip protocol

Zebra: Fix setting source for 5549-learnt routes via ip protocol

Ticket: CM-6854
Reviewed By: CCR-3297
Testing Done: bgpsmoke, bgpclos to verify setting source (in 2.5-br)

Two pieces prevented the user from specifying a route-map with set src on
IPv4 routes learnt via BGP's RFC 5549 model (v4 prefix with v6 nexthop):
   - There was code missing in the section specific to 5549 in setting
     the src in the netlink message
   - During RIB processing, route-map processing was ignored when the NH
     was v6 and the route itself was v4.

As per the code, all route-map processing that uses nexthop validates the
NH type before applying the route-map and so there should be no errors
as a consequence of relaxing bullet 2 above.

Signed-off-by: Dinesh G Dutt <ddutt@cumulusnetworks.com>
Reviewed-by: Donald Sharp <sharpd@cumulusnetworks.com>