routing: populate fibs with interface routes after growing net.fibs.
Currently it is possible to extend number of fibs in runtime, but this
functionality is of limited use when net.add_addrs_all_fibs is
non-zero, as the routing tables are created empty.
This change automatically populate newly-created fibs with the kernel-originated
interface routes (filtered by RTF_PINNED flag) if net.add_addrs_all_fibs
is set.
```
-> sysctl net.add_addr_allfibs=1
net.add_addr_allfibs: 0 -> 1
-> sysctl net.fibs
net.fibs: 2
-> sysctl net.fibs=3
net.fibs: 2 -> 3
BEFORE:
-> setfib 2 netstat -rn
Routing tables (fib: 2)
AFTER:
-> setfib 2 netstat -rn
Routing tables (fib: 2)
Internet:
Destination Gateway Flags Netif Expire
10.0.0.0/24 link#1 U vtnet0
10.0.0.5 link#1 UHS lo0
127.0.0.1 link#2 UH lo0
Internet6:
Destination Gateway Flags Netif Expire
::1 link#2 UHS lo0
2a01:4f9:3a:fa00::/64 link#1 U vtnet0
2a01:4f9:3a:fa00:5054:ff:fe15:4a3b link#1 UHS lo0
fe80::%vtnet0/64 link#1 U vtnet0
fe80::5054:ff:fe15:4a3b%vtnet0 link#1 UHS lo0
fe80::%lo0/64 link#2 U lo0
fe80::1%lo0 link#2 UHS lo0
```
Differential Revision: https://reviews.freebsd.org/D36075
MFC after: 1 month
(cherry picked from commit
40503b792f7ecb9fcb349b2519036f85fa62fb4c)