Currently, the table id for VRF devices requires an integer. Convert
it to use rtnl_rttable_a2n which handles table names from the iproute2
directory.

This also fixes a bug in the original commit where table name are not
properly handled.

Fixes: 15faa0a3 ("add support for VRF device")
Signed-off-by: David Ahern <dsa@cumulusnetworks.com>