net: Add SKB DMA mapping helper functions.

Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/core/Makefile b/net/core/Makefile
index b1332f6..26a37cb 100644
--- a/net/core/Makefile
+++ b/net/core/Makefile
@@ -6,6 +6,7 @@
 	 gen_stats.o gen_estimator.o net_namespace.o
 
 obj-$(CONFIG_SYSCTL) += sysctl_net_core.o
+obj-$(CONFIG_HAS_DMA) += skb_dma_map.o
 
 obj-y		     += dev.o ethtool.o dev_mcast.o dst.o netevent.o \
 			neighbour.o rtnetlink.o utils.o link_watch.o filter.o