userspace TSO: SCTP checksum offload optional.

userspace TSO: SCTP checksum offload optional.

Ideally SCTP checksum offload needs be advertised by the
NIC when userspace TSO is enabled. However, very few drivers
do that and it's not a widely used protocol. So, this patch
enables SCTP checksum offload if available, otherwise userspace
TSO can still be enabled but SCTP packets will be dropped on
NICs without support.

Fixes: 29cf9c1b3b9c ("userspace: Add TCP Segmentation Offload support")
Signed-off-by: Flavio Leitner <fbl@sysclose.org>
Signed-off-by: Ilya Maximets <i.maximets@ovn.org>