net: Set rtm_table to RT_TABLE_COMPAT for ipv6 for tables > 255

Set rtm_table to RT_TABLE_COMPAT for ipv6 for tables > 255 to
keep legacy software happy. This is similar to what was done for
ipv4 in commit 709772e6 ("net: Fix routing tables with
id > 255 for legacy software").

Signed-off-by: Kalash Nainwal <kalash@arista.com>
Signed-off-by: David S. Miller <davem@davemloft.net>