interface: don't store table id in route if the default table is used

interface: don't store table id in route if the default table is used

Use system_is_default_rt_table() to test whether the resolved routing table
is the default one, in this case do not apply the table attribute to the
route object.

This is needed for backwards compatibility - only routes using a non-default
table shall report it in the ifstatus output.

Signed-off-by: Jo-Philipp Wich <jow@openwrt.org>