erspan: get the proto with the md version for collect_md

In commit 20704bd1 ("erspan: build the header with the right proto
according to erspan_ver"), it gets the proto with t->parms.erspan_ver,
but t->parms.erspan_ver is not used by collect_md branch, and instead
it should get the proto with md->version for collect_md.

Thanks to Kevin for pointing this out.

Fixes: 20704bd1 ("erspan: build the header with the right proto according to erspan_ver")
Fixes: 94d7d8f2

 ("ip6_gre: add erspan v2 support")
Reported-by: Kevin Traynor <ktraynor@redhat.com>
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Reviewed-by: William Tu <u9012063@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>