net: Fix routing tables with id > 255 for legacy software

Most legacy software do not like tables > 255 as rtm_table is u8
so tb_id is sent &0xff and it is possible to mismatch for example
table 510 with table 254 (main).

This patch introduces RT_TABLE_COMPAT=252 so the code uses it if
tb_id > 255. It makes such old applications happy, new
ones are still able to use RTA_TABLE to get a proper table id.

Signed-off-by: Krzysztof Piotr Oledzki <ole@ans.pl>
Acked-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>