net: Revert "fib_trie: use seq_file_net rather than seq->private"

This reverts commit 30f38d2f.

fib_triestat is surrounded by a big lie: while it claims that it's a
seq_file (fib_triestat_seq_open, fib_triestat_seq_show), it isn't:

	static const struct file_operations fib_triestat_fops = {
	        .owner  = THIS_MODULE,
	        .open   = fib_triestat_seq_open,
	        .read   = seq_read,
	        .llseek = seq_lseek,
	        .release = single_release_net,
	};

Yes, fib_triestat is just a regular file.

A small detail (assuming CONFIG_NET_NS=y) is that while for seq_files
you could do seq_file_net() to get the net ptr, doing so for a regular
file would be wrong and would dereference an invalid pointer.

The fib_triestat lie claimed a victim, and trying to show the file would
be bad for the kernel. This patch just reverts the issue and fixes
fib_triestat, which still needs a rewrite to either be a seq_file or
stop claiming it is.

Signed-off-by: Sasha Levin <sasha.levin@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>