lwtunnel: get nlsize for erspan options properly

erspan v1 has OPT_ERSPAN_INDEX while erspan v2 has OPT_ERSPAN_DIR and
OPT_ERSPAN_HWID attributes, and they require different nlsize when
dumping.

So this patch is to get nlsize for erspan options properly according
to erspan version.

Fixes: b0a21810

 ("lwtunnel: add options setting and dumping for erspan")
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Reviewed-by: Simon Horman <simon.horman@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>