compat: Allow IPv6 GRE/ERSPAN Tx when ip6_gre is loaded

compat: Allow IPv6 GRE/ERSPAN Tx when ip6_gre is loaded

When for some reason the built-in kernel ip6_gre module is loaded that
would prevent the openvswitch kernel driver from loading.  Even when
the built-in kernel ip6_gre module is loaded we can still perform
port mirroring via Tx.  Adjust the error handling and detect when
the ip6_gre kernel module is loaded and in that case still enable
IPv6 GRE/ERSPAN Tx.

Signed-off-by: Greg Rose <gvrose8192@gmail.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>
Acked-by: William Tu <u9012063@gmail.com>