xfrm: Convert xfrm_bundle_create() to dscp_t.

Use a dscp_t variable to store the result of xfrm_get_dscp().
This prepares for the future conversion of xfrm_dst_lookup().

Signed-off-by: Guillaume Nault <gnault@redhat.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>