xfrm: Override skb->mark with tunnel->parm.i_key in xfrm_input

This change makes it so that if a tunnel is defined we just use the mark
from the tunnel instead of the mark from the skb header.  By doing this we
can avoid the need to set skb->mark inside of the tunnel receive functions.

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