bgpd: fix aggregate route display

bgpd: fix aggregate route display

Based on RFC-4760, if NEXT_HOP attribute is not
suppose to be set if MP_REACH_NLRI NLRI is used.
for IPv4 aggregate route only NEXT_HOP attribute
with ipv4 prefixlen needs to be set.

Testing Done:

Before fix:
----------
aggregate route:
*> 184.123.0.0/16   ::(TORC11)               0         32768 i

After fix:
---------
aggregate route:
*> 184.123.0.0/16   0.0.0.0(TORC11)          0         32768 i
* i                 peerlink-3               0    100      0 i
*                   uplink1                                0 4435 5546 i
   184.123.1.0/24   0.0.0.0(TORC11)          0         32768 i
s> 184.123.8.0/22   0.0.0.0(TORC11)          0         32768 i

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