[IPV4]: Fix single-entry /proc/net/fib_trie output.

 
When main table is just a single leaf this gets printed as belonging to the 
local table in /proc/net/fib_trie. A fix is below.

Signed-off-by: Robert Olsson <robert.olsson@its.uu.se>
Acked-by: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: David S. Miller <davem@davemloft.net>