netlink/route: provide pre-2.6.19 Linux compat shim

The old Linux used 8-bit rtm_table field of the RTM_NEWROUTE message to
specify routing table id. Modern netlink uses RTA_TABLE 32-bit attribute.

Unfortunately, there is modern software (namely bird) that would prefer
the old API as long as the routing table id fits into 8-bit.

PR: 279662