[IPV4]: Move trie_local and trie_main into the proc iterator.

We only use these variables when displaying the trie in proc so
place them into the iterator to make this explicit.  We should
probably do something smarter to handle the CONFIG_IP_MULTIPLE_TABLES
case but at least this makes it clear that the silliness is limited
to the display in /proc.

Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: Denis V. Lunev <den@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>