Currently 'ip route get' does not show the table the lookup result comes
from and prior to kernel commit c36ba6603a11 the response from the kernel
was hardcoded to the main table. From the discussion this appears to be
a leftover from the route cache where the cached entry lost the table id
and so the result was hardcoded to main table.

c36ba6603a11 added the RTM_F_LOOKUP_TABLE flag to maintain that behavior
but to allow new tools to ask for the actual table id for the lookup.
This patch adds that flag to ip route get request and if the result is
not the main table shows the table id.
Signed-off-by: David Ahern <dsa@cumulusnetworks.com>