ipfw kfib algo: Use rt accessors instead of accessing rib/rtentry directly.

[melifaro]

Summary

Use rtentry accessors defined in D27416 to avoid direct rtentry field access.
This removes assumptions on prefix storage and rtentry layout from an external code.

Test Plan

2:44 [0] m@devel0 s kldload ipfw
22:44 [0] m@devel0 s ipfw table 10 create algo addr:kfib
22:44 [0] m@devel0 s ipfw table 10 list
0.0.0.0/0 0
10.0.0.0/24 0
10.0.0.157/32 0
127.0.0.1/32 0
::/96 0
::/0 0
::1/128 0
::ffff:0.0.0.0/96 0
2a01:4f8:13a:70c:ffff::/96 0
2a01:4f8:13a:70c:ffff::6/128 0
fe80::/10 0
fe80::/64 0
fe80::5054:ff:fe42:fef/128 0
fe80::/64 0
fe80::1/128 0
ff02::/16 0