backports: ifdef code already present in RHEL7.0

Avoid conflicts with RHEL7.0 as DEVICE_ATTR_* defines,
enum pkt_hash_types, skb_set_hash(), dma_common_get_sgtable()
are already present.

Signed-off-by: Stefan Assmann <sassmann@kpanic.de>
Signed-off-by: Luis R. Rodriguez <mcgrof@do-not-panic.com>