datapath: Add missing code in ip_tunnel_lookup()

datapath: Add missing code in ip_tunnel_lookup()

The compat rpl_ip_tunnel_lookup() function was missing some code added
in Linux kernel release 4.3 but not backported in the initial commit.

This also allows us to remove an old hack in erspan_rcv() that was
zeroing out the key parameter so that the tunnel lookups wouldn't fail.

Fixes: 8e53509c ("gre: introduce native tunnel support for ERSPAN")
Reported-by: William Tu <u9012063@gmail.com>
Signed-off-by: Greg Rose <gvrose8192@gmail.com>
Acked-by: William Tu <u9012063@gmail.com>
Signed-off-by: Justin Pettit <jpettit@ovn.org>