zebra: Fix off by one error in no vni command

zebra: Fix off by one error in no vni command

Commit: e2bcfa3946aadc62af53cf53ff8d2f9fe7e5bab1

Introduced a off by one error in the `no vni XXX` command:
@@ -2337,12 +2336,10 @@ DEFUN (no_vrf_vni_mapping,
        "VNI-ID\n"
        "prefix-routes-only\n")
 {
-       int ret = 0;
        int filter = 0;
-       char err[ERR_STR_SZ];
-       vni_t vni = strtoul(argv[2]->arg, NULL, 10);

        ZEBRA_DECLVAR_CONTEXT(vrf, zvrf);
+       vni_t vni = strtoul(argv[1]->arg, NULL, 10);

        assert(vrf);
        assert(zvrf);

Fix it.

Fixes: #6867
Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>