We currently don't do TSO if ip options are present. In case of IPv6, we look at
in6p_options to check that. That is incorrect as we carry ip options in
in6p_outputopts. Also, just checking for in6p_outputopts being NULL won't
suffice as we combine ip options and ip header fields both in that one field.
The commit fixes this by using ip6_optlen() which correctly calculates length
of only ip options for IPv6.

Reviewed by: ae, bz
MFC after: 3 weeks
Sponsored by: Limelight Networks