bgpd: flowspec redirect vrf uses vrf table instead of allocated table id

bgpd: flowspec redirect vrf uses vrf table instead of allocated table id

Until now, when bgp flowspec entry action was to redirect to a vrf, a
default route was installed in a specific table. that route was a vrf
route leak one. The process can be simplified, as vrf-lite already
has a table identifier. Actually, because policy routing is used to
redirect traffic to a defined table (with ip rule command), use
the table identifier of the VRF.

Signed-off-by: Philippe Guibert <philippe.guibert@6wind.com>