zebra: fix flowspec ipset operations

zebra: fix flowspec ipset operations

When injecting an ipset entry into the zebra dataplane context, the
ipset name is stored in a separate structure. This will permit the
flowspec plugin to be able to know which ipset has to be appended with
relevant ipset entry.
The problem was that the zebra dataplane objects related to ipset entries
is made up of an union between the ipset structure and the ipset info
structure. This was implying that the two structures were on the same
memory zone, and when extracting the data stored, the data were incomplete.

Fix this by replacing the union structure by a defined struct.

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