[IPV4] fib_trie: size and statistics

Show number of entries in trie, the size field was being set but never used,
but it only counted leaves, not all entries. Refactor the two cases in
fib_triestat_seq_show into a single routine.

Note: the stat structure was being malloc'd but the stack usage isn't so
high (288 bytes) that it is worth the additional complexity.

Signed-off-by: Stephen Hemminger <stephen.hemminger@vyatta.com>
Signed-off-by: David S. Miller <davem@davemloft.net>