erspan: Check IFLA_GRE_ERSPAN_VER is set.

Add a check to make sure the IFLA_GRE_ERSPAN_VER is provided by users.

Fixes: f989d546

 ("erspan: Add type I version 0 support.")
Cc: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: William Tu <u9012063@gmail.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>