fib_trie: Cleanup ip_fib_net_exit code path

While fixing a recent issue I noticed that we are doing some unnecessary
work inside the loop for ip_fib_net_exit.  As such I am pulling out the
initialization to NULL for the locally stored fib_local, fib_main, and
fib_default.

In addition I am restoring the original code for flushing the table as
there is no need to split up the fib_table_flush and hlist_del work since
the code for packing the tnodes with multiple key vectors was dropped.

Signed-off-by: Alexander Duyck <alexander.h.duyck@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>