ipv4: fib_select_default should match the prefix

fib_trie starting from 4.1 can link fib aliases from
different prefixes in same list. Make sure the alternative
gateways are in same table and for same prefix (0) by
checking tb_id and fa_slen.

Fixes: 79e5ad2c ("fib_trie: Remove leaf_info")
Signed-off-by: Julian Anastasov <ja@ssi.bg>
Signed-off-by: David S. Miller <davem@davemloft.net>