zebra: move the check for l3vni

zebra: move the check for l3vni

The two checks for l3vni have been already done in
`lib_vrf_zebra_l3vni_id_modify()` as it should be. And it is improper that
the two checks are put after `zebra_vxlan_handle_vni_transition()`, which
will do real things.

My original fix is to remove them. But NB module can't guarantee many changes,
so we'd better keep them in `zebra_vxlan_process_vrf_vni_cmd()` in APPLY stage
for safe.

Just move them in front of `zebra_vxlan_handle_vni_transition()`.

Signed-off-by: anlan_cs <vic.lan@pica8.com>