bgpd: Fix import check removal

bgpd: Fix import check removal

Fix: 06e4e90132ad23815c6f288dd7e6be334f5ab233

Modified BGP to pay more attention the prefix returned from
zebra to ensure that a LPM wasn't accidently causing BGP
import checks to think it had a match when it did not.
This unfortunately removed the check to handle the route
removal.

This sequence of config and events would leave BGP in a bad state:
ip route 100.100.100.0/24 Null0
router bgp 32932
  bgp network import-check
  address-family ipv4 uni
    network 100.100.100.0/24

Then if you removed the static route the import check would
still think the route existed:

donatas-pc(config)# ip route 100.100.100.0/24 Null0

donatas-pc(config)# do sh ip bgp import-check-table
Current BGP import check cache:
 100.100.100.0 valid [IGP metric 0], #paths 1
  blackhole
  Last update: Sat Apr 23 22:51:34 2022

donatas-pc(config)# do sh ip nht
100.100.100.0
 resolved via static
 is directly connected, Null0
 Client list: bgp(fd 17)

donatas-pc(config)# do sh ip bgp neighbors 192.168.10.123 advertised-routes | include 100.100.100.0
*> 100.100.100.0/24 0.0.0.0                  0         32768 i

donatas-pc(config)# no ip route 100.100.100.0/24 Null0

donatas-pc(config)# do sh ip nht
100.100.100.0
 resolved via kernel
 via 192.168.10.1, enp3s0
 Client list: bgp(fd 17)

donatas-pc(config)# do sh ip bgp import-check-table
Current BGP import check cache:
 100.100.100.0 valid [IGP metric 0], #paths 1
  blackhole
  Last update: Sat Apr 23 22:51:34 2022

donatas-pc(config)# do sh ip bgp neighbors 192.168.10.123 advertised-routes | include 100.100.100.0
*> 100.100.100.0/24 0.0.0.0                  0         32768 i
donatas-pc(config)#

Fix this by moving the code to handle the prefix check to the
evaluation function and mark the bnc as not matching and actually
evaluate the bnc.

Signed-off-by: Donald Sharp <sharpd@nvidia.com>