ipv6: sr: remove SKB_GSO_IPXIP6 on End.D* actions

ipv6: sr: remove SKB_GSO_IPXIP6 on End.D* actions

After LRO/GRO is applied, SRv6 encapsulated packets have
SKB_GSO_IPXIP6 feature flag, and this flag must be removed right after
decapulation procedure.

Currently, SKB_GSO_IPXIP6 flag is not removed on End.D* actions, which
creates inconsistent packet state, that is, a normal TCP/IP packets
have the SKB_GSO_IPXIP6 flag. This behavior can cause unexpected
fallback to GSO on routing to netdevices that do not support
SKB_GSO_IPXIP6. For example, on inter-VRF forwarding, decapsulated
packets separated into small packets by GSO because VRF devices do not
support TSO for packets with SKB_GSO_IPXIP6 flag, and this degrades
forwarding performance.

This patch removes encapsulation related GSO flags from the skb right
after the End.D* action is applied.

Fixes: 1c696cdc0319 ("ipv6: sr: add helper functions for seg6local")
Signed-off-by: Yuki Taguchi <tagyounit@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>