ip_vti/ip6_vti: Do not touch skb->mark on xmit

[ Upstream commit cd5279c1

 ]

Instead of modifying skb->mark we can simply modify the flowi_mark that is
generated as a result of the xfrm_decode_session.  By doing this we don't
need to actually touch the skb->mark and it can be preserved as it passes
out through the tunnel.

Signed-off-by: Alexander Duyck <alexander.h.duyck@redhat.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>
Signed-off-by: Sasha Levin <sasha.levin@oracle.com>