Prior to this commit, the VXLAN "create" API assumed
the vrf_id belonged to only the IPv4 FIB tables space.
With this commit, the FIB table is found in either the
IPv4 or IPv6 table as determined by the is_ipv6 flag.
This follows the same pattern that was already being
done in the CLI code for the VXLAN "create" command.
Change-Id: I35d5e37db24efa858e4696dc2c004fa64bb4a4a8
Signed-off-by: Jon Loeliger <jdl@netgate.com>