zebra: default ns->info should be default zebra_ns

zebra: default ns->info should be default zebra_ns

We were not connecting the default zebra_ns to the default
ns->info at namespace initialization in zebra. Thus, when
we tried to use the `ns_walk_func()` it would ignore the
default zebra_ns since there is no pointer to it from the
ns struct.

Fix this by connecting them in `zebra_ns_init()` and,
if the default ns is not found, exit with failure
since this is not recoverable.

This was found during a crash where we fail to cancel the kernel_read
thread at termination (via the `ns_walk_func()`) and then we
get a netlink notification trying to use the zns struct that has
already been freed.

```
(gdb) bt
\#0  0x00007fc1134dc7bb in raise () from /lib/x86_64-linux-gnu/libc.so.6
\#1  0x00007fc1134c7535 in abort () from /lib/x86_64-linux-gnu/libc.so.6
\#2  0x00007fc113996f8f in core_handler (signo=11, siginfo=0x7ffe5429d070, context=<optimized out>) at lib/sigevent.c:254
\#3  <signal handler called>
\#4  0x0000561880e15449 in if_lookup_by_index_per_ns (ns=0x0, ifindex=174) at zebra/interface.c:269
\#5  0x0000561880e1642c in if_up (ifp=ifp@entry=0x561883076c50) at zebra/interface.c:1043
\#6  0x0000561880e10723 in netlink_link_change (h=0x7ffe5429d8f0, ns_id=<optimized out>, startup=<optimized out>) at zebra/if_netlink.c:1384
\#7  0x0000561880e17e68 in netlink_parse_info (filter=filter@entry=0x561880e17680 <netlink_information_fetch>, nl=nl@entry=0x561882497238, zns=zns@entry=0x7ffe542a5940,
    count=count@entry=5, startup=startup@entry=0) at zebra/kernel_netlink.c:932
\#8  0x0000561880e186a5 in kernel_read (thread=<optimized out>) at zebra/kernel_netlink.c:406
\#9  0x00007fc1139a4416 in thread_call (thread=thread@entry=0x7ffe542a5b70) at lib/thread.c:1599
\#10 0x00007fc113974ef8 in frr_run (master=0x5618823c9510) at lib/libfrr.c:1024
\#11 0x0000561880e0b916 in main (argc=8, argv=0x7ffe542a5f78) at zebra/main.c:483
```

Signed-off-by: Stephen Worley <sworley@cumulusnetworks.com>